To answer some of your questions:
Romainian prodaters work differently than ukrainian ones, who use "agencies" and "translators."
They do them themselves, with help from their own family, friends. Money, flowers, dates in other countries, even houses to live in etc. are "gifts" to them, and hard to prove otherwise. Unlike the Ukrainian pro-daters, they will hang on until the victim is entirely cleaned out of all money, and they are tenacious in attacking anyone posting their names and profiles online as scammers.
Is it common for a woman to approach a man on a fan forum rather than a dating site?
Yes, it is. Scammers are everywhere online. It is also possible that this one saw another profile of his elsewhere, then approached him on the fan site, where her approach would be more subtle and easier to hook him there.
I wish I had trusted my intuition earlier when I could have maybe had an influence and done something about it.
I'm afraid that even then, you would have had no chance to change his mind about her. I have seen her FB, and looking as gorgeous as that along with the tall stories and promises of fame and riches that he desired which were eluding him, you would have been completely unable to change his mind. Most all men would fall like flies, I'm sorry to say.
can't find any trace of her. I still know about her face book
Scammers will move and delete profiles to remove traces of their activities. We cannot post her FB account here without direct victim proof of being scammed, and you will never ever get him to provide any emails, etc. or any proof that she scammed him.
You need to stop "chasing the scammer." Stay away from her, your friend being scammed, and anyone else involved in it. You are a victim in this too, and putting this behind you and moving forward is your only way to recover from this.
Don't read about Pro-daters at online sites right now, ok? Everything you need to know about them has been answered by firefly and myself already here. If you keep looking you will spin "in the scam" forever, and not recover from it.
It seems like you are trying to second-guess yourself, either to assign blame to yourself for not knowing enough about them or not doing the right thing. Beating your self up this way is not helpful for you. You had zero chance to stop it, regardless of what knowledge you would have gotten online beforehand. There was also nothing online posted that she was a scammer from anyone else, either, so he would dismiss anything you had to say about her.
Spend some more time offline, do some things outside, away from the pc, and any online "scam-related activity" that you do should be about recovery issues, not about the scammers themselves and how to fight them.
We have podcasts with romance scammed victims that would help you for a start.http://scamsurvivors.com/podcasts/