Need some advice about a situation.

You can delete if not allows here.My friends(24M) side chick (25f)has expressed that she wants have sex with me after finding out the my friend has other girls besides her. She know he has been doing stuff with other girls but she still loves him and does not love her back he has told me this in person.

The reason he dont take her seriously because she is a single mom and has a crazy baby dady. He is just using her and going to dump her when he finds a girl he actually like. Now to the main topic of discussion. She is beautiful and we get alone just fine, when she asked me if I would like to have sex with her I said I cant because the “bro code” he has my back and I got his I told him he got a little mad at her. Now I told him that was the hardest thing I had to do because im not popular with the ladies so her just asking me if I would like to is like winning the lottery for me.

I am very tempted but keep turning her down. Now my friend just kinda fucked me over because we were gonna go on vacation but he canceled because he promise this other girl that he would watch her dog. I chose him over the one thing I want most but he chooses a dumb dog over me. Now we we planning on going on a cruise we had done everything except buy the tickets. Now I feel hurt because I was really looking forward to the cruise and dont know what to do.
Tl;dr my friends side chick asked if I woukd like to hook up with her and I said bro code and didnt now I regret after what my friend did.