Your girlfriends husband is with another woman!

I would talk to the husband privately and ask him if he's having an affair with another woman. If he denies it, tell him that you're keeping an eye on him. Do a little snooping around just to confirm everything. If it's true indeed that he's having an affair, tell your friend about it. She's gonna be hurt i know but she has every right to know that her husband is cheating on her.

I would certainly not tell my girlfriend. You get involved in domestic disputes and when they get themselves sorted, or not, you end up being the bad one. I might confront the husband at some point, just to get his response, after all, it could be innocent. But never, ever, tell on someone, its just way too dangerous.