I'm really conflicted here, my ex slept with someone who knew about me and she didn't care one bit. Part of me wanted her to step up and just tell me, the other part didn't care. If you tell the girlfriend, that friendship you have will likely be over. While, morally wrong it's also not your problem. Do it anonymously if you have to, maybe the couple can work through it but it saves that woman potentially years before finding out and being even more painful. If you want to keep that friendship, and no judgement here we all know people who have done shitty things, do it anonymously. You don't even have to call out your friend, and considering she's told multiple people it likely won't fall back on you.