I should have picked up on (trying this post from another computer)....geeze that is another one I have to add to the red flag list!
There is also ( upon re-reading his post again... ) the hint that he thinks he is to be part of a ffm threesome with you and your girlfreind and is getting mixed signals...or perhaps just really really wants it and is looking for any hint of a signal and reading even the slightest nice "hello" as a signal.
May be ( if she is willing) a little threesome could quell his ego at not being as marketable as you. ( I agree with your analysis of shy vs gregarious...but...would add that often shy people seem to think others should "come to them".)
All of this may be his brusied ego at how well you are doing and are recieved, his jealousy over it triangulating into an affair with a woman who is playing to that weakness... Adulterous individuals tend to look for that opening...( I read the damn website from the other link too... it sucks...but they do it.)
Just free thought here hon... hoping to make sence and help and get her the hell out of the pic and you back to hunky dory
take what makes sence and works and disguard the rest.