Ex husband

Sarah

So my ex husband just told me he's dating my best friend of 7yrs. She "dates" about 6 men a month ever since I've known her. He knows this. Needless to say I have stopped talking to her altogether and his and my relationship is very rocky right now. We have two small children together so coparenting is a must. I'm wondering if I'm over reacting or under reacting? I feel very betrayed by both of them.