I get that she was upset about the second bite, especially since it was more severe than the first. If she felt she didn't want to chance another biting encounter she should have been up front with you about how she felt. She could have let you feel better, because as a friend, she would know you felt horrible about the incident, by acknowledging that this can happen to anyone and she doesn't blame you yet needs to take her son early from your care because she doesn't want to risk a third bite. I had a biter as well and it can happen so quickly even if you are deligent. I quickly learned his behaviour leading up to his biting so I could stop him before he offended. Let her make the next move in your "friendship"