Child's dad talking down about mother??

So I babysit this 6 year old girl 6 days a weeks, every night she calls her dad. She calls him on Skype so she can see him and I don't mean to eaves drop but sometimes I can't help it. Today they were talking about costumes and she said she wanted to be peter pan. He proceeded to tell her how her mom is only letting her dress up as a boy because she wants her to be a boy. He went on to talk about how irresponsible she is for allowing their daughter to wear a boys costume. I wanted to defend her mom but I didn't want to be rude and interrupt them. What should I do?