Do a little digging and find out who OM's wife is. If you know his name, public records may well reveal her name and other info.
In the meantime, tell your W that next weekend doesn't work for you, maybe sometime the following week.(you are out having fun on the weekend, of course). This will buy you a bit of time to try to contact OM's wife and find out what OM has been telling her. If OM is lying to his W, then you have a chance to expose the affair.
Until you see what else you can find out, say NOTHING to your W about what you know and how you know it.
