idiom requests - "He is an opportunist, there's no need to give him more excuses or opportunities!"

We have a saying in my country:

He doesn't need music to start dancing. He is already dancing without music!

Which figuratively means:
He doesn't need any special, real, or necessary excuses for taking advantage of a situation or to interfere in other people's affairs. He uses the smallest pretext for his unwanted intervention/taking advantage. Let alone you encourage him with more excuses or opportunities.

The saying expresses one's disapproval: He is ready to use any excuse, even the smallest one, for doing something, and this behavior really bugs me! This sarcastic observation is in fact criticizing the person's behavior. And we're also warning the listener not to give that person any excuse.

Here are two scenarios which illustrate the saying:

A: Suppose that my son doesn't like studying math, he continually postpones doing his assignments, and uses any excuse to avoid doing them. For example, his father asks him to help with some chores in the garage when he has lots of math homework to do! The next day he goes to school with his unfinished assignment, and when I ask for an explanation, he replies: "I had to help Dad!"; in this instance I might scold my husband like this:

"He already dances without music! Why did you give him an excuse to escape from doing his assignment? You should have asked for his help after he had done his assignment."

B: My mum has diabetes, sometimes she doesn't follow her diet plan and eats sweet food in secret. Whenever there is a family gathering, everybody feels sorry for her and tells me: Don't tell her not to eat this, not to eat that, so often! Let her eat anything she likes just this time! At which point, I reply:

"Sorry, but you don't know something, she is already dancing without music!

by saying this, I mean that she is already eating everything she wants (secretly), let alone I actually give her permission to ‘bend the rules’!

(Actually when we say "he dances even without music", it implies that he knows how to dance, even without the music, so if you play music for him, he will dance better.-[he knows how to take advantage even there is a small chance, so if you provide him with more chances, he would/might take even more advantage.]

Question: Is there any word, term, idiom, expression, saying, quote, or proverb that conveys the same meaning, when talking about someone with this * opportunistic* character?