I mean for what, he didn't steal anything, or anything that the police would consider a crime, but you should have a talk with your daughter of what you have heard, and say something like "Grandpa told me what did while he was in the pool, and he dud not like that, and so I am telling you and please promise me that you will never do it again". It is up to you to decide if you want to tell her, that he asked you to intervene if she does it again. But really you kicked your own father from a party for defending himself, and he decided to tell you of what she did, but either cause your daughter should apologize to your father of what she did to him in the pool. If your daughter knows what she did is wrong, then there is hopefully a very good chance that she will never do this again, even when you nip it into the bod. Your father might need anger management issues, but you do need to get this problem solved sooner than later.