Bit of background. My exh and I split up after 16 years together (9 years married) when I found he was sleeping with a work colleague (prior to this we had a very good marriage and everyone, including myself, was shocked) We subsequently divorced and he is still with OW. They have our 3DC EOW & 4 weeks per year hols, and although I know the importance of having their father in their lives, the fact that the OW who distroyed my family gets to play happy families with my DC absolutely kills me.
DF is someone I have known since nursery (30+ years). We lost touch during our teens but got back in touch around 9 years ago. We don't see each other that often but have 3 or 4 days out together every year, plus occasional nights out with partners (I'm now remarried) and we frequently FB message. She is my oldest and one of my closest friends.
DF only knew Exh through me. A couple of years before we split, Exh got DF's partner a job in his firm. They didn't work directly with each other but saw each other in passing and we would have occasional nights out as a foursome. Neither my Exh or DF's partner work for the same company anymore as both left some time ago now.
It has come to light recently, via FB, that DF and her partner have being seeing ExH and his OW socially. OW is now listed as a 'friend' of DF and frequently comments on DF's posts, making it obvious that they are more than just acquaintances.
I feel very hurt that DF of years saw everything I went through because of this woman but now she is friends with her. I've not said anything to her because I can't obviously tell someone who they can and can't be friends with, but aibu to feel betrayed and upset? DF has messaged me to ask to arrange a night out with our partners but I haven't replied because I don't know what to say. The OW took my husband, plays 'mum' to my children and now has taken my friend too. 
Would you lose an old friend because of OW or should I just accept that she can be friends with whoever she likes, even if it does hurt?