What to do?

[Sugar and Spice]

1) If I tell his W she may go nuclear and push him out, which will cement my wife and OM.

Only if he doesn't want his M. If he has no intention of losing his family over this, then he will dump her.

2) I know he mentioned to my W that he thinks she should try to reconcile, as it would be best for her (although he'd clearly like to maintain some fringe benefits)

That tells you what you need to know. He is not going to run off with her, shes a good time...thats about it.

3) I could always contact his wife later, since I have proof.

Or pland the seeds of suspicion now.

I fear though that even with exposure of this relationship, W would just seek a new relationship, rather than come around to me. Heck, she will probably harbor some hatred towards me (for ruining her plans, and to cover her guilt)

Yep, but this is just going to be her way of justifying what she is doing. Its not about you, this is about her and what SHE is lacking. Don't forget that.

Ah it's hard. I've been planning to have a talk with W on Sunday night, and still don't know what to do.