Sorry if this is really long I just don't want to drip feed info.
So my bf and I have been close since primary school and are like family. About a week and a half ago she split with her DP of 6+ years. I had seen it coming as they weren't spending much time together, were arguing a lot and didn't seem to be making any plans for the future despite being together for years. But I was surprised one of them finally had that balls to do it, tbh.
This weekend she asked to see me again and confided that the real reason was that she is in love with one of her colleagues in her new job which she's been in since the end of the summer and the feeling is mutual. Apparently nothing had happened prior to her split with DP and out of the blue her colleague asked if she split with DP because of him 
she admitted yes and they spent all weekend discussing their feelings over text.
Now the thing that really gets me is that her colleague has a long term gf. So he's been discussing his love for my friend while his poor gf is oblivious. I asked if anything had happened between them and she said no apart from the discussion. But I just don't believe that? I don't see how anyone could be so bold as to ask a colleague if the end of their relationship was because of them without something already happening between them?!
About a month ago she went on a night out with the colleague and some of his friends from uni and in the lead up to it she mentioned it to me quite a few times but now looking back I realise she never mentioned it after the fact. Which is unusual as she would normally give me a ring to let me know how she got on especially as it was a new group of friends. Now I'm thinking something must have happened on the night out which is why she suddenly went quiet about it?
I also think there must be something more to the story because she didn't tell me she liked this guy before now which is so strange as we have always told each other everything.
My problem is now I feel very suspicious of her as I'm sure she isn't being truthful and that they've been carrying on behind their DP's backs. I truly don't condone cheating I think it's an awful thing to do and even if truly nothing bar the conversation has happened I still feel uneasy that she's entertaining this guy while he still has a gf. Surely on his part they're beginning an emotional affair?
If it was anyone else I'd just draw back and mind my business but as we're so close she keeps phoning and texting me asking for advice and wanting to chat about the situation and I feel so uncomfortable with it all. I can't stop thinking about his gf. I'm also wondering what kind of person my bf is now.
This summer I found out my long term partner had been cheating on me and my bf was my rock throughout that time. She cried with me when I found out, helped me pack his stuff and enthusiastically ranted with me about both him and the OW so she's seen first hand what being cheated on can do to a person and now she's taking part in it herself. Regardless of if something physical has happened I'm sure their conversation would be enough to devastate his gf.
My question is AIBU to be so disgusted by the situation? And what would you do if you were me? I don't want to lose a friend but the longer this goes on the more my opinion of her will lower and I definitely don't want to come across as supportive of the whole thing either. I would never judge someone for their feelings, that can't be helped but I definitely do judge acting on those feelings when one of you has a partner.