just let you know, there's sth called peer influence. people, especially teens, are very likely to be influenced by the thoughts and behavior of their peer. this could be what is happening to this friend you are talking about.
to be honest, this is an issue between you and your friend, and now you are including a third person in it. i mean, it's understandable that you want help from common friends to sort this out, but it doesn't always work. it is, after all, a problem between you and your friend and it should be solved between you two.
i think what you can do now is leave both of them alone for a good while of time, but still remain kind and polite to them. for example, greet them whenever you see them, and maybe have some small/short conversations, but DON'T keep going on and on, that will make them think that you are annoying.
i have been through this, and in my case, almost all my friends suddenly hated me and i have no idea why (i believe that it's because i annoyed them too much). i know there's nothing i can do in that period of time, so i leave them alone to calm down, and miraculously after a few months before the school year ends, they started talking to me again and we are still very good friends now (this happened 6 years ago). i believe what happened is that they have calmed down from whatever made them angry, and they saw that i am not how they thought i were, so they accept me again!
i understand that it's hard to go through this. what i did was to focus on other thing, like music (i love music). whenever i play music, or even just listen to music, it makes me forget everything i worried about. also, being a Christian, i go to church and that is the most comforting place for me in that period of time!^^
如果你係對佢好嘅,我想你應該順其自然,多给一點空間別人,當他悶悶不樂時,你就盡量去鼓勵他,如果佢仲係唔理你,佢可能好忙,for example:girl friends,
bad friends and so on.你可以嘗試問佢有乜煩惱,令佢知你係好朋友