My friend (44M) and his ex (40F) share a daughter (13F). The daughter has a male friend (13M) and she insists they're just friends and not dating.

The daughter wants to have sleepovers at her friend's house. My friend is completely against this. The ex wife let's her do it and they already had a sleepover behind my friend's back. He only found out later about it. The girl of course is fully on her mother's side and acts like her dad is old fashioned. I don't think he is , I think he's being reasonable. Is there anything he can do ?