Should i be upset with her?

Your pal and your ex maybe had interest prior to your break up and now that you have broke up it was his get of jail to jump with your friend, sounds awfully suspicious that after you delete on facebook she jumped at the chance, you friends seems a bit of a cow tbh, and your ex seems a cold hearted bastard who just thinks with his dick (Shock horror, its in our male genes lol) Dont let it affect you and just get on with life, i am sure you will find a nicer bff and bf