AITBF for defending my daughter’s guest at a family gathering?

We had a family gathering today with lots of extended family. My daughter (14F) invited a friend (14F) to come over to hang out, eat, and have fun.

All was going well but I noticed the friend who was looking happy and upbeat suddenly looking upset. She tells me her mom was coming to pick her up early. I asked her if she was okay and what had happened. She stated that my nephew ((8M) my husband’s side) was picking on her and being mean over using a slip and slide. The girl felt bad and quit doing the slip and slide and that was when she sat down with me and had her mom come get her. I don’t know the friend very well but she seemed to be mentally challenged but she was a sweet girl.

I didn’t have a chance to speak with the mom before they left because it was all of a sudden. My MIL was the one throwing the shindig and she wanted to know what happened. I specifically said I was not there when it happened but this is what Friend told me. I told MIL that it hurt her feelings and she left early due to nephew saying things. Nephew is the golden child.

MIL immediately became defensive and saying Nephew wouldn’t say that and it was just kids being kids. I told her that Friend was a guest at our house and I felt bad for the girl. She starts raising her voice and basically saying it couldn’t be true and that everyone was a guest. I again told her that I was not there and I don’t know what happened but that was why Friend left early. This was the first time Friend had ever been over there. I told her if we couldn’t talk it out rationally that I was walking away so I did. I went for a walk to cool my head.

When I came back I was told I shouldn’t have said anything, I ruined the peace, should have kept my mouth shut and that the little girl may have lied. We left the get together and went home. I haven’t heard from the mom and my daughter hasn’t heard from Friend. I feel bad for the girl. She was even telling me how she is bullied at school. Now we have her over and this happens.

Am I the buttface? Should I have just kept my mouth shut and not said anything?

tl;dr: at a dinner daughter’s friend says my nephew was being mean so she had her mom pick her up early. MIL asked why and I told her what I was told. I was accused of causing drama and I should have kept my mouth shut and not defended the friend.