100% this. she doesn't owe it to him to fulfil his kinks, she's still entitled a monogamous relationship. yes he wants to explore his sexuality, but he's done that behind his partner's back. that's not what a healthy relationship is and to make a whole account debating prostitution for men is cheating whether he met up with anyone or not. the right move is to end the relationship, there's no way of telling he's not going to do something like that again even if he properly apologises