They’ve got enough going on. They don’t need this on their plate. They’ve got honeymoons to enjoy, boxes to unpack, and the ill fated thank you cards to get carpal tunnel syndrome over. The last thing they need is an affair gone awry on their schedules. Do NOT tell the couple. Settle it on your own or amongst the groomsmen that know all the parties involved, but will be sworn to secrecy as you figure out how to mitigate this situation. Also, do not engage the maid of honor about it. That’s like when criminals return to the scene of the crime. Have a friend or another groomsmen approach one of the other bridesmaids to keep all parties independent and away from each other.