OP you are a legend. Strong fabulous woman we should all strive to emulate in horrific, traumatic times.
He wants to know how you know because that will allow him to assess the degree of damage that has been done, and the strength of his current position regarding finances and his business.
Do NOT give your evidence away as that will allow him to make this assessment. You have all the power for now.
Do not contact the OW either...that gives her some power. Don't give her the time of day. Can't get her own man.
He is the ultimate asshole (and I blame him primarily), and if she did plant evidence how embarrassing for her. Spineless, skulking around the bathroom to tuck a boarding pass in. LOSER, can't stand up and be counted for her own actions.
You deserve so much better and you will have so much better.
Btw, you have had a long marriage which is not a failure just because he has slipped away recently. You have had a very successful relationship which has produced your gorgeous children. Just because he has lost himself at 58, does not mean any past failure. Time for a change. Same happened to my parents after 33 years of marriage and my mother has accepted that her marriage was a success, just wasn't meant to last forever. She is now madly in love with her new husband.