Its dangerous as you’re asking what YOU should do, but your mate IMO should have a word with both his GF and the goon. There’s nothing wrong with saying how you feel and saying that you’re experiencing a bit of jealousy isn’t always a bad thing as it shows you care.
With the goon and I would talk calmy and firmly so that he expressly new how I was feeling about the situation. Then if it came to it i’d get the bombers out (sorry but I love this phrase now).
I still stand by (as others have said) that its a mine field to get involved with their relationship though, especially if you’re friends to both. I would really stay out of it unless you see/hear something, then you can get involved.