Sorry to have hit you with a list of question's like that, wasn't trying to pry, there just wasn't a lot of info from your first post.
So he was into thing's in the beginning, then as it went on he became distant with her? with you? You did not mention how your friendship is affected.
It is possible he has had a renewed interest in her. Either from his own adventures or from seeing her in a more happy mood. Maybe seeing the way's you make her happy and he want's to do the same, like old dog learning new trick's? The J word is always an option as well.
Having her reject his advances to the point of demanding him to go see other people is something which makes me wonder about the future stability of their relationship. What may have started as one thing, is becoming something different. My best advice is to discuss what is going on, and expectations, both of them. Alone, and as a group if possible. There is an age gap here so assumptions may be different amount everyone.