What should I tell his gf⁉️

I was sleeping with this guy I thought was single and my bff found his Instagram and he’s all in a relationship . But ok Facebook he’s single . I want to tell the girl but I’m scared to. What do I do ? I haven’t told him I know but I haven’t talked to him in 2 days since I found out. I’m mad and confused and I don’t want to message her out of anger but she deserves to know . What would I even say?

I was in a 6 year relationship before this and it ended badly so I decided to be single and just have sex with one person and me and the guy been hooking up since March . And my friends like no way he hasn’t made a move to be your bf yet unless he has a gf and sure enough he did .

😪