[Mrluvaluva]

Tough one. Your friend could react either way. If you don't tell them, and they find out you knew, they may be mad at you for not telling them. If you do tell them before the wedding, they may be mad at you for trying to spoil things and may refuse to believe it anyway (especially if their partner refutes your claims). Either way, you could lose a friend. I think I would tell them, and then they had the choice to proceed how they wanted. If they held my actions against me and wanted nothing to do with me afterwards, then they weren't such a good friend after all.