I wouldn't feel good about it. Maybe if the ex turned up out of the blue and they went out for a beer and she couldn't reach her hubby but left him a note, that'd be more reasonable...but an ongoing friendship they kept secret, not. IMHO.
Do you feel uneasy when you think about this choice of hers, and this response?
If you do feel uneasy, do you respect that feeling?
Is it just a feeling wiith no meaning, or is it a warning system?
How do you feel when you imagine yourself as her spouse one day? Would you be worried there might be another exBF in the bushes? If so, you okay with that?
Bottom line: what kind of relationship/partner are you looking for? The clearer you can get on this, the happier your life will be. (Take it from someone who waited too long to decide character comes first, then chemistry...that I'm alone in middle age.)
Me, I would've rationalized it away when I was younger, since I was so needy I made endless excuses for people. IMO, you don't have to judge her harshly or hate her. No satisfaction in that really. But you might want to do some sober thinking about how much you value yourself.
Hops