Call me Mr. Insensitive but... I can't believe your still seeing your ex- when she is with another guy. This sounds as if your still burning a bit of a light for her, I always found that cutting the ties and moving on works best, hard as that may seem. But if you were (are?) that passionate about the other one still then hanging out once a month may not be the best thing. Maybe a little time down the road of life you meet again and then maybe a friendship develops. Be a good friend to your ex-mate's ex-. She needs someone who has been hurt by him too, you are on the same wavelength. Maybe together you can get over his betrayal of you both.

Alistair