Is there an English equivalent to the Syrian proverb: “Throw meat to the dog, and he'll forget the bread and salt”?

Question

When they throw meat to the dog, he forgets the bread and salt (companionship one had with someone else).

This Syrian phrase is said to someone who throws away years of friendship and gregarious company with someone or people, and betrays, abandons or leaves them etc.

Imagine being with someone who you've shared many meals with; you have both been through the good and the bad, the happy and bitter times, simply everything. Arabs value companionship like no other. It is considered shameful if someone leaves his longtime companion when the going gets tough, or when he is abased, when he is down after he was at the top. We say he is a man of companionship, he who honours friendship and really cares.

Is there anything like this in English especially pertaining the second part 'forgets the bread and salt'?

• Bread is referring to the bread they broke.
• Salt can refer to the bad they went through together somehow. It's a cultural thing.

Answer 1

You might call such a person a fair-weather friend:

One who is friendly, helpful, or available only when it is advantageous or convenient to be so.

(https://en.wiktionary.org/wiki/fair-weather_friend)

Answer 2

Someone who would make this kind of a "switch" will "trade you in for a new model."

The new "model" was originally a reference to a car, but could mean new friend, lover, whatever.