It means she should slow down and not put so much energy into her work or she will use up all her energy. She can't continue at the fast / intense rate. If you are driving, you can "ease up on the gas", which means to slow down. If the boss is angry and starts yelling at the workers, he should "ease up" and not be so severe with them. If you are at a party drinking too many alcoholic drinks, someone might tell you "you should ease up a little (on that). You have to be at work early tomorrow morning."

It's sorta slang it usually means someone is telling you to "chill out" and not to worry as much

used in certain conditions such as insulting someone "ease up" means to calm down and stop insulting them or when driving it means to slow down or stop driving so agressively. basically it means stop and think before what your about to do.