The number one red flag with me is not respecting limits, and ignoring rules (yes, from Dom side!).
Such behaviour, if accepted to begin with, will get worse and worse over time.

I had a contact with one person here who was exactly like it. I put up with the behaviour because I enjoyed the play with him so immensely (during that play, he was an altogether different character!), and there was a girl with him whom I also liked very, very well.
After a while it got too much, though, and my warnings were either not taken seriously, made fun of, or outright ignored.
In order to end it, I completely broke off all contact. To keep him away, I also had to break off all contact with the girl, which made me very sad as she was a very sweet, likeable person, and did not act as obtrusive as him at all. But as she was his prior play-partner, there was of course no possibility to break contact with him, but not her.

Anyways. The things I am talking about included: Ignoring my friendly explanation that I am doing only ONE conversation (on messenger or wherever private) at a time. If I said good-bye and went to another convo, he would keep nagging, babbling, even BUZZING, and would not stop.
He would bad-mouth any other man I liked talking to.
My number one rule from hubby (my real life hubby and only real life Dom) is that I am allowed to play online, but no real-life contact is allowed whatsoever... I explained that repeatedly... and still the other guy tried to force a dinner meeting in real life upon me (this one really pissed me off for good).
What really creeped me out was that he was ALWAYS set in invisible mode on msn - ever, at all times. And combined with this was that he not only left roughly 20 offline messages of the really stupid sort DAILY (along the lines of "I am bored", "where are you" etc... for that matter, I was at WORK, could you imagine????), but also messaged me NO MATTER WHAT TIME I checked in, and tried to keep me out of any other convo with the actions described above. I began to get the impression that he had no job, no life, no friends, and never slept, because he really was online every single time I checked in over a period of several weeks.
When I finally broke off contact (explanations would not help, as he decided to pretend not to understand what creeped me out), he tried for a period of several weeks to get messages to my attention, using other subs on here as messengers (direct contact was impossible as I had blocked him from my profile).
The one thing I was glad about was that he kept this obsessive behaviour online. He never tried to contact me in real life or at work or wherever, and I am very glad about that.

OK, I feel I am getting agitated about this. My tips for your friend would be to watch out for the following signs and attitudes:
1. ignoring set rules (like, no real-life contact or giving out of personal location details)
2. asking for a lot of personal info without giving any out himself (like asking for your job and location, but refusing to reveal anything about himself)
3. acting dumb when you express unhappiness with a current situation
4. not leaving you alone when you request it.

Oh, and I completely agree about the only-chatting and not-contributing-anything-helpful in the forums issue, completely.

Kind regards
Arria