No reason to be mad at him, he was up front about what he found out. There is a reason to be annoyed with the ex and with the sister. But, it's up to him to make it clear to his sister that this isn't acceptable and won't be tolerated. He should clue in the entire family in no uncertain terms that this has to stop. If the ex has been as persistent or stalker-like as he says, he will have to get nasty with her and absolutely refuse all contact to put an end to it and even then it she may continue to try - especially if his sister is encouraging things. If you like the guy, give him a break and see if things work out in the long haul.