Tbh I wouldnt unless they do get back together, which sounds unlikely. From what you have said, she has got him sussed and will tell him to sling his hook. In the unlikely event that they do get back together then I would monitor the website and if he is still online alot then I would tell her then.
I'd tell her but partly because my bf went through years of a shitty relationship and no one who had seen the cheating first hand told her. I had suspicions but No proof. My bf is very hurt that no one (ie people who were her friends but not best friends) told her the truth.
I would if I was her friend but the OP said that they arent friends as such, more "exes in common" as it were. And he could use that against the OP to imply that she is shit stirring because she is jealous or bitchy or unhinged...
I would wait until she knows for sure that they are back together, and then I might just get the best friend to exchange messages on the dating site with him so she can prove to the fiancee what a shit house he is.
I'd steer clear. I was matched to a friends boyfriend of 7 years! I told him to man up and either split with her or stay with her but take down his profile and at least treat her with some respect! The messenger will be shot!