Darrel*
There are two very huge scumbags we have identified in this brief text exchange. The first scumbag is the ex-girlfriend, who tried reaching out to the OP about getting back together. Obviously, his answer was no, and he should’ve blocked her right away.
The second scumbag is Darrel, whom the OP knew on a personal level. It’s bad enough to learn that your ex-SO cheated on you with someone you know, but to cheat on you with someone who’s engaged? This guy really needs to find a new circle of friends to hang out with. We seriously feel sorry for the OP.