with gf's mum - what shall i do?

Tell your friend! Friends are supposed to keep there friends from getting hurt. Yes, what you tell her may hurt her feelings but it will potentially keep her from a world of different hurt caused by her freak of a BF. Who would in there right mind who had hopefully tight young 25 year old puss sleep with a 48 year old saggy wrinkly dried up meat curtain for a puss? Looks like someone has issues with intimacy and will bang anything that walks so I'm uming the 48 year old is not the first nor last girl he will cheat on your friend with! Seriously... you'd do your friend a favor!!!!