How to ask/tell her nicely?

Just say "I'd prefer if we could just stick to her regular feed" (especially if you are buying this feed yourself.) Then ask if she would mind your hanging up a sign that says "Please do not feed '____'" so that the other boarders don't feed her unauthorized treats. You don't have to be confrontational -- just polite and matter-of-fact. If she IS your friend, you should be able to ask for her cooperation. That she charges you so little really is irrelevant.