What to do about a biter I babysit?

I babysit a two-year-old girl regularly and she has started constantly biting my baby. I always put her in time out when it happens but it doesn't help. The girl's mother doesn't do anything about the biting when she is around, just usually ignores it, so I feel like the problem won't go away since she is not being disciplined at home. I don't feel like I can punish her physically since she is not my child. Woudl it be immature of me to tell the mother I will no longer babysit her until she stops biting? Or what should I do?