Nope! Don’t say a word. The relationship between your friend and her soon to be fiance is really between the two of them. It’s really inappropriate for you to get in the middle. However, if you’re uncomfortable knowing what you know and being in the middle, you have a couple of options.
First of all, choose one of the two of them to be friendly with — in this case, it should really be your girlfriend. It’s too hard to hear secrets from both of them. So let go of the friendship with her boyfriend and become more of “her friend”.
Second, you can encourage her to be truthful, if that’s what you think is the right thing (because it’s good to be honest with your friend about who you really are), but respect her decisions and don’t push or cross her. If her betrayal keeps you from being friends with her, then let go of the friendship, but if you can understand that people make mistakes, and the one she made is one she recognizes as a mistake, then look for your humanity and listen to her and support her being her best self.
Third, keep the proposal to yourself. Keep her betrayal to yourself. These are white lies for you that do less harm if kept than told.
I hope that helps. Let me know how things go, and please follow me @AskAprilcom on Twitter and on Facebook at this link: [url][/url]. 😀