@terifairy,
It sounds more like to me, given you know the things she is doing, including turning up at a bowling alley that you are fully "aware" or made "aware" of the goings on.. So, I wouldn't call that cheating, rather she's a nutter and won't get the hint.
People handle things differently, we are not all the same.
Do you have any reason to think that he would cheat on her with you seriously?
If, not, allow him to handle it his way... Realtionships are built on trust and without trust, there is no relationship.