I don't think so, because if he did, then cheating tag would have to be added.
Right now, MC catches his girlfriend cheating in the past and dumps and leaves her right there. If she randomly shows up and wants to be with MC again, then I think the cheating tag would have to be added.
Overall, if the scene wasn't about the girlfriend's sex with another guy, then rather MC catches them before they end up doing anything/ walks in hearing them talking about it. Then I wouldn't mind having MC's ex-girlfriend get redeemed.
To be personal, I love the render of the girl, but being willing to cheat on MC without talking to him or other women before spreading her legs is a no-go for me because she is clearly an idiot and just wants sex.