Give somebody an inch and they'll take a mile.
something that you say which means that if you allow someone to behave badly at all, they will start to behave very badly I'm always wary about making concessions to these people. Give them an inch and they'll take a mile.
-From the free dictionary.

This is something you can work with, if you think that is the problem. But talking with her face to face primary first, like testing the battle ground, before jumping into a conclusion might be a better idea.

Unfair can be an emotion than a principle. It can be your weakness in certain circumstances.