Guys who are into anal are into causing pain, if she's up for it, fine. If not, that's her call. As for the "getting on top" thing, well that should be up for negotiation.
But guys who stray are looking for something that they're not getting at home and if he's out putting it in strange places, he's going to bring home something besides flowers, if you catch my drift.
Having said that, however, there's not much you can do other trhan make it clear that you're there to be supportive if and when she decides to leave the guy.