Some older person was talking about evicting some renters from her property, and she said to me “she had a snake in her ear”.
This is an allusion to the Biblical event of Eve being persuaded by the devil in the form of a snake to disobey God. A bad actor or perhaps just bad thoughts (as if they came straight from the devil) were at play when she made her decision to evict the renters from her property.
I found some examples of the expression being used as a much closer allusion to the Biblical story, or even with reference to the story itself. From a mass:
Everything was good, but then but then along came the snake whispering sweet deception into her ear: Eve. Hey Eve. It’s me hey Did God really say that you can’t eat from any of these wonderful trees? No. No, he didn’t say that.
No, she Heisman’d me, dazzled by the other snake whispering sweet Apple in her ear. Pretty good deal it turns out; trades damn near two hundred bucks a share now.
The book Secrets in the Attic:
I had my own Garden of Eden at my house. [...] Was there a snake whispering in our ears?
A Google Books search turns up several recent instances in which "a snake in [one's] ear" seems to function as a metaphor for being susceptible to bad thoughts, bad advice, or a bad attitude toward God.
From Daughter of Sarah, From Adam to Jesus: What Every Wife Should Know (2014):
A wife must be Jesus centred – her beauty comes from focusing on Jesus (beauty coming from the heart/centre/Jesus) and then His beauty will manifest in her. We must not put our trust in our own beauty, but focus on the beauty of Jesus to influence and touch our husbands. This beauty is eternal and will capture your husband. This beauty from Jesus is manifested as a quiet and gentle spirit. So when your husband speaks to you in a disrespectful manner you are given two choices either eat from the tree of knowledge of good and evil or The Tree of Life. You will most probably hear the voice of the snake in your ear pushing you to eat the fruit of the flesh:
Are you going to let him speak to you like that? That i not hoe the Bible prescribes him to treat you. You need to tell him that he is not fulfilling his role as a husband in the eyes of God. Until he decides that he is going to treat you with respect you will also not honour him as your leader.
From Sara Gennusa, Pearls from the Heart (2019):
I will not make agreements with a stranger's voice neither in word nor deed. I will guard my heart and my tongue against such agreements. I will only confess Your words, for out of the heart spring the issues of life. "Keep your heart with all diligence, for out of it spring the issues of life" (Proverbs 4:23). I will not feed the snake in my ear. I will consciously awaken my heart and starve him out with Your word, Jesus, and Your word alone. The trap has been set and I am anticipating sweet freedom.
From R.S. Gunn, A Certain Woman: Accepting Your Call and Meeting the Conditions (2020):
Awesome. [According to my physician] I could have an occasional drink. And I did. Then my occasional drink turned into a fairly often drink. I felt good there. I could have a glass of wine or cold brewski every few days. That wasn't so bad. God wouldn't punish me for that!
I was actively eating the fruit.
I wasn't listening to God; I had a snake in my ear, and I liked what he was saying.
But the serpent said to the woman, You shall not surely die. (Genesis 3:4 AMP)
In all three instances, the snake evidently represents an external, diabolical source of personal motivation—as the serpent in the Garden of Eden does.
An older but obviously kindred expression replaces snake with serpent. For example, from Edward Bulwer Lytton, The Wife's Tragedy, in Poems (1855):
Gertrude. Yes. I see it all at last— / All in ruins. I can dare / To gaze down oe'r my lost past / From these heights of my despair. / O, when all seem'd grown most drear— / I was weak—I cannot tell— / But the serpent in my ear / Whisper'd, whisper'd—and I fell.
From Brian Edwards, Daddy Was a German Spy (2008):
We walked and Barry [who said that he had been "saved" three years earlier] talked. He was so joyous it made me want to put my finger down my throat. We arrived at the ABC cinema on the Lisburn road. The movie was Picnic, starring Kim Novak and William Holden. I'd seen it the previous Saturday and lust had not slept since. Barry looked at the poster of Novak dancing seductively in the band rotunda.
'Well, brother,' he said, in a presumptuous display of fraternity. 'you'll never be in that den of iniquity again.'
His voice was drowned out by the hissing of the Serpent in my ear: 'Never see Kim Novak again, Brian! Never again!'
I was a lost soul.
From Gerald Wood, A Life of Jesus, by the Good Doctor: Meditations and Reflections on the Gospel of Luke (2017):
"The devil said to him, 'If you are the Son of God, command this stone to become a loaf of bread'" (Luke 4:3, 4).
If you look at the temptation that Jesus faced here, it is all so reasonable. Jesus was hungry. Bread is a basic staple of life. What would be wrong to turn a few rocks into bread? God would understand. The most basic of all temptation is not to place our trust in God when it seems we have to turn our back on our most basic responsibilities. God is not calling us to be outwardly religious while inwardly self-serving. It is why we never reach a point beyond temptation. Yes, the lurid and sensational may not be attractive to us, but that does not mean we are ever free from the serpent in our ear asking, “Did God really mean that?"
And from Simon Chesterman, I, Huckleberry (2020):
My hand gripping the wet stone, I climb onto the ledge.I am directly above the clock face: eight minutes past eight, as it always has been, and as it always will be.
"That's right Huck," The voice is a serpent in my ear. "“It will all be over soon. You do know how this ends after all."
Once more, I look down at the courtyard, its cobblestones waiting to embrace me.
And then I saw that the true end was indeed the death of the author. An end to sorrow and confusion, to suffering and embarrassment. The serpent in my ear told me to jump and I climbed onto the ledge.
But there was an interruption, another voice, another story to be told.
Of the preceding quotations, only one takes the exact form that the poster asks about: "I had a snake in my ear." But all involve using the phrase "snake in my ear" as a figurative (or supernatural) embodiment of temptation, evil counsel, or justification for sinful behavior. It seems fair to say that for some people, anyway, "having a snake in one's ear" means playing host to an advocate for unrighteous behavior.
Whether the landlady cited in the poster's question was saying that one of the renters she was evicting had fallen under the influence of wicked tendencies is impossible to say from the information provided in the question. But while using the figure of a snake in one's ear to represent such a person is hardly idiomatic or proverbial in English, it does appear in some published works and would certainly be recognizable to many English readers as an allusion to the biblical story of Eve, the serpent, and the tree of the knowledge of good and evil.