Don't know what to do

Last night, my SO and his daughter's mom got in a fight because he thinks there's something going on over there. His daughter has come over several times with clothes that smell like pee, finger prints, several giant bruises, and its even looked like she was drug across the floor at one point. She has also told us she sits on her step dads face after she tried to sit on her dads face and he told her no. Her mother then blocked him, and unfriended me on Facebook. I don't want him to not be able to see her anymore, but I feel like she's going to try her hardest to make that happen because he's a behind in child support (he didn't start paying until she was 3-4, and they start it from when the child is 2), and we don't see her as often as we want because we live in a different state an hour away, he works most of the time, and I'm a full time student.

I need advice step mommas.