It depends on her attitude. Coffee can be expensive or cheap. If she has the heart to do her job well, I don't mind treating her coffee for breakfast. If she takes it for granted, I will ask her to buy it for herself when she has got the salary.
If you want to build up a good relationship with her, which is very important, you may have to be "generous" on "small matters", but when it is about job performance, you should set boundaries.