I need some help please. My boyfriend has been helping take care of a child who his ex says may not be his.?

My boyfriend has known this since the child was born but decided to step up to the plate and be there for the kid. The kid is now 11 years old. My boyfriend has been going thru hell by the child mother over money for many years. Just yesterday on Father Day we drop the kid off after spending the day and she came out to the car an punched my boyfriend in face because he didn't have money to give her for camp. I can understand her being upset if she know that this was his child, but she herself said that she doesn't know and my boyfriend is the only man out of 3 that spends time with the kid. This is starting to affect my relationship with him because I am tired of her mistreating my boyfriend and me having to listen to it. I can't even go with him to pick up the child anymore becuase of how she might act. He loves that kid so much and she knows it and uses that to hurt him. I've advise him to take a paternity test, but at this point even it turns out not to be his he can't just cut the kid off. But at the same time how does he get away from the drama if don't leave the situation along. Please help.