Probably because she was ‘available’...Did they already move in the same social circles I wonder?
I know it’s awful to say-but there are many older women ‘out there’ who are lonely and fed up of living alone. Along comes a smiling man who pays them some flattering attention, and vice versa- and before you know it, the thrill of an affair becomes intoxicating. They don’t pause to think of the devastating consequences on the guy’s wife. In the moment, they won’t even care...
Glam yourself up girl so he can see that you’re younger/more appealing!!! Act like you don’t need him. Be successful and aloof. Have an air of mystery about you and see him come creeping back out of curiosity/missing you, when he’s sick of her!