Do I tell the girlfriend/fiance that he is cheating?

I was dating a guy for over a 1/2 a year and just recently found out that he has a long-term girlfriend, who I think he may be engaged to. I had NO IDEA and broke off all communication with the guy immediately upon finding out. He lied about everything. But, now I am trying to decide whether I should tell his girlfriend/fiance. I don't want it to be done out of spite or revenge. I want to do the right thing here. He has cheated on her before me and I know he has been (lately) also looking for other women to have nsa relationships with online. Should I let her know before she goes and makes a lifelong commitment to this guy? For me, in the long run, I think I'd want to know. Part of me wants to just walk as far away from this guy and never turn back, but part of me thinks I should at least attempt to open her eyes (if they aren't all ready opened). Do I tell?