He can't be your best friend and also not like at the same time.
Howevever if "he is dangerously jealous" it's probably a good idea to stay out of it. No doubt his lady is aware of his temper and has chosen to put herself at risk.
If the lady is your best friend then your loyalty is to her. You are better off creating some distance between you and her. She may be using you as an alibi. Cheaters generally at some point draw their friends into their drama.
If you told him she might claim you came onto her and once she turned you down you came up with this story. If he doesn't like you because you are a man there is a good chance he will believe her over you. There is no "upside" for you either way.