bestfriend advice

Elli
Hey girls I need some advice, so basically my bestfriend of like 8 years or so has fallen for someone I have had past relations with (not a relationship specifically but we definitely had feelings for each other and this was quite recent, about 2 months ago) and she has asked if she can act on it. I have made it absolutely crystal clear from the beginning that it makes me very uncomfortable that she has been talking to him and that if she were to do anything in that sense it would definitely make me unhappy. She has brushed this off and continued talking to him saying they're just friendly intentions and that she doesn't at all find him attractive and that he is actually 'ugly'. Well now she's admitted she has fallen for him and has asked for permission to take things further with him. What should I do about this? I see her face everyday at school and I just have all of this hate and hurt inside of me but I don't know how to act on it as if I were to drop her as a friend it would cause big problems within our group.