Is it ok to date or fool around with a friends ex?

Friends had this discussion recently, one of my buddies wants to start dating a friends ex…the friends ex is from 5 years ago and he was also dating someone else at the same time. Though emotions were involved and might even still be there is this wrong?

Might also want to mention that the guy and girl who are ex’s still talk to this day, not sure if that factors in but im sure they might still have some kind of emotions for each other.