Thanks Fidelia,I appreciate your input. I discussed it at length with him last night and he is adamant that he is not going to give into her demands for monogamy when he knows he can't do it. He also admitted their dependence is what I described as 'toxic' and he is finding it very hard to withdraw on an emotional level but not physical.
For this reason I suggested he take time out for himself,give himself time to heal and let me just be his friend. Although I have feelings for him I would much rather he is healed for the sake of ANY future relationship. So I have organised(along with another one of his female friends) something for him for his birthday to let him know he has our unconditional support,because we all were friends before this happened.