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if I were you I would track her and the next time she is at a carpark dogging or whatever she does turn up tap on the window and at least be man enough to knock the other guy clean out and walk away.
the disrespect he has shown to you certainly can not go unrewarded.
I bet the thing is the other guy doesn't want to take her on anyway and he's just using her for sex, as she said
he clearly does not actually want her as an actual partner just someone he can bed when he feels like it and she is a big enough fool to let it happen for 7 years.
alternatively you could turn up and give him your blessing , shake his hand and say "good luck to you both" your missus will be shocked as hell

the other guy likely doesn't want her anyway as I already said

sit back and enjoy single life