Friend sleeping with the same man

Advice needed. Been sleeping with a man for the past 5 months. I told my best friend after about 3 months and she told me that her and him had history, i didnt know they had as she has been with her current partner for years but she had cheated on him. The past few weeks shes been acting weird and shady. I just caught her and him at it. Iv pretended to both i didnt. Im hurt as i have feelings for him. Do i tell them i know. Do i confront her and just cut him off. I dont really know what to do my heads a mess.