I have this friend....?

That I really care about (more than he'll ever know)... and he was seeing this one girl for a month but she dumped him for an ex and it really hurt him, I mean more than I really thought and then after about a month of being with her ex they ended up breaking up and literally like a day later they ended up back together, which worries me. I don't trust her and I do not want to see him getting hurt again but was this a stupid decision on his part? Do you think it will last? I don't want to intervene because I don't want to get in his business I just need answers...