Don't be so sure that UA will simply let OP off the hook if he notifies UA. It's still a clear contract violation and UA may simply cancel the tickets.

At this point, I would simply fly the OW and hope that UA doesn't have tickets flagged by this fraudulent operation for review.

After you have flown, you may want to help out your fellow travelers by posting on social media. Some poor slob is going to show up at a ticket counter and be denied a boarding pass.