I think if all this would be for her is sex, and you need more time to feel safe in your relationship with her again (I mean, hell, it sounds like you were on the road to divorce!), it's perfectly reasonable to ask her to desist with this guy. Give it 6 months, see if you feel different then. Again, why push it if it's just a roll in the hay for her? It would be different, obviously, if she was in love or something.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.