Well, there are two possible scenarios here. The first scenario is that he still has feelings for his ex and is possibly still banging her out. But it is ENTIRELY also possible that he really is upset with his "bro" because dude is in clear violation of the G-code.
Rule #2 states that you do not sleep with your homeboy's woman at any time, ever. New York's drug cartel, The Commission, headed by Nicki Barnes, was felled because of group members sleeping with other men's women. For guys, that's the ultimate form of disrespect for some reason. Of course, it only matters if you really cared for her and it usually reserved for wives. I remember one time I was hanging with my friends and my boy was like, "Yo, P, you know you my dude right? Like straight up, I wouldn't even bang your girl!"
The odd part is that he meant it as way of saying how much he valued me as a friend. Guys are just weird like that. It's not even about you all, it's about the respect we assign to our friends and what boundaries and lines we won't cross under any circumstance. Real G's do real things. And real things doesn't involve sleeping with another man's woman.
Realize though, if true, scenario two - though described as a possibility in depth - does not negate the fact that he could very well still want to schlump his ex and is jealous. But if every conversation he has goes like, "I can't believe HE'D do that" then he might not be lying about the homeboy transgression. And if he mentions the code, he's being honest. Dumb, but true.
You should just pay attention. If he's still got feelings, he'll tell you even if he doesn't say it explicitly. Men do that too. Believe me, he'll let you know.
It was written.
Tell your man to put on his big girl panties and deal with it! He's not with her any more, so why the heck should it even matter? Feel free to quote me.