Replying to I think I lost my best friend. What do I do?

I'll do my best to make a long story short. my best friends gf cheated on him. No one told him for months. I told him a few months ago after I found out . I thought he should know the truth. He thanked me and told me he's handle it from there. Well. Turns out his gf found out I told him and she began to hate me, and she also convinced him to stay with her. Since then. He always picks his girl over his friends and he basically refuses to talk to me because "my gf hates you and says I shouldn't be around you because you make her want to cut herself". I ignored it for a while but finally told him I thought it wasn't cool and that you shouldn't do that to your friend. I've known this guy forever and he's like a brother to me but now he's treating me like a stranger. I'm ignoring him as of now because I've basically given up.

I know it's a stereotype to say "I hate drama" but honestly I hate this shit. I try to stay out of it but whenever I try to talk to him it's like I get pulled into it. His gf texts me saying shit like "you make me want to kill myself . You think im a whore" to which I say "please stop texting me" which then she goes and complains to my friend and he further ignores me. It's pretty childish and I really wish it'd stop but she keeps it going.

Worst part is though, were supposed to go to Colorado in a month. I haven't spoke to him in a few weeks and Im not sure how I'm going to handle the trip. I stay out of his way and when I talk to him I try to be as chill as possible but he just acts so weird and runs back to his girl.

So. What should I do? Ignore him further til the trip? Cancel the trip? Stop talking to him all together ? Idk who else to ask I don't usually ask advice but I really am confused.

Tl;dr my best friend got cheated on, I told him, his gf hates me as a result, he stayed with her, now because of her he refuses to talk to me, and we're supposed to go to Colorado in about 3 weeks.