Your senior coworker was correct. Opinion would be a suitable word in that situation and discretion would not.
In general, when you ask for someone's discretion, you are in effect asking them to keep something private (just between you and them). In other words, your conversation is to be kept a secret, at least until the person who is confiding in you says later that it's now OK for you tell others.
Discretion can be used in other ways, too. When you ask someone,
Please use your discretion in talking with others about the upcoming layoffs at the company,
you are asking them to use their best judgment, which could mean, for example, they limit the number of people they tell about what might happen.
Here is a quotation from Shakespeare's play, "Henry the Fourth":
Falstaff: "To die is to be a counterfeit, for he is but the counterfeit of
a man who hath not the life of a man; but to counterfeit dying,
when a man thereby liveth, is to be no counterfeit, but the true
and perfect image of life indeed. The better part of valor is
discretion, in the which better part I have sav'd my life" (Part 1 Act 5, scene 4, 115–121, my emphasis).
Here, when Falstaff uses the word discretion, he is saying in effect that while valor (bravery) is a good thing, sometimes simply choosing to live another day, instead of dying as a hero, reveals discretion (good judgment).
Put differently, Falstaff is saying that as good as valor might be, sometimes using one's discretion to stay alive is far better!