You should listen to him and enquire from his friends about it. However, you do not have to do that if you do not want him back. It could be that he regrets what he did and that's why he is asking for forgiveness.
Not rely on words. Anyone who would date your so called best friend after he broke up with you is immature, thoughtless and possibly vengeful. Any best friend that would date him is not your best friend. Dump them both. 5 years from now or maybe even less, it won't matter.
I give him credit for waiting to date your best friend after you broke up. But dating your best friend is just not okay. You deserve better than that. I'm sure he does still "love" you, but oh well. He sure wasn't thinking about how much he "loved" you when he was with your best friend.. Move on.
You down times have to think like a man. He probably loves you so much and thought that by dating your best friend what would hurt you most. Hear his side and go from there and see how ling they were dating. if it was long then you should forget about it because he could've dated you to get close to her, if it was short then he was definitely trying to make you jealous and hurt. The one I'm worried about would be the "best friend". Everyone knows you don't date any of your friends ex.
It depends on how you feel about it. If him dating her really hurt you, don't go back. If he left you for your best friend, definitely don't go back. However, people fall for people that might upset someone. If they dated a month or so after you broke up, and they really liked each other a lot, I wouldn't hold it against them. Feelings are weird. They mess with us. So it just depends on what the situation was and whether or not it was a serious dick move on his part. If he's not a good guy, don't do it. But if you really want to and the whole thing was just him falling for the wrong person, he's not a bad guy and he might be genuine.