@hedonista, if the 'counselling' continues, you need to get in and be part of his solution to the girl's "problem". I guess you can apologize and make lambing and understand the situation and that you want to help and promise him you won't do get mad again, hoping he will let you in the 'counselling/comfort/emergeny etc.
I had a similar situation, bf had a close officemate, since they see each other daily at work. Before, work related lang usapan, technical questions via text messages on weekends. I had no problems since I have met the girl before, she's not ugly but also not strikingly beautiful, but she was slim but also flat-chested. When I saw her again, I saw that she changed, like an ugly duckling into a swan, I guess she was blooming for him. A few months, i sense bf cold shoulder towards me, less dates, no time to accompany me, he'd rather be busy at home even when i visited him at home. I could not pinpoint yet who but since i know his other ofcmates, i asked them to watch over him for me since he's been cold and might be hot for someone else. I already had an idea who but I got the confirmation when one officemate got defensive towards me.
Anyways, to make it short, bf was courting the girl na, I know his moves since he courted me noon, same tactics lang he used on the girl, and the girl kunwari wala siyang alam at wala siyang kasalanan, but gossip spread quickly. All our reputations were actually affected. That bf cheated, na si girl mang-aagaw & desperate, and she said that i'm just a paranoid, possessive, jealous, insecure gf for thinking there's something between them.
Girl denied everything to me, nilaglag nya si bf by dumping him and putting the blame on him to save her face, by her actions she really looked guilty to me. I wasn't able to do anything to her, it was bf fear that i would hurt her or make a scandal. I just told bf na, hindi nya ako kaaway, we are a team, and that we can still work things out if he wants.