Teppo -- no judging here. Ask yourself if you took your son for a walk in that location because you deep down hoped that you would run into her with OM. It's a big world with lots of places to walk, so this seems an odd coincidence.
If you DID do this to potentially create a confrontation, mission accomplished. Sometimes we feel a deep need to push things to a head, sometimes as much to prove to ourselves what's really going on versus any other reason.
Unfortunately, what you may have learned, is that it can be a lot like bullying -- it feels good in the moment but afterwards you feel much worse.
I agree with everyone else that your job should be to shield S as much as possible from what's going on, but I'm sure you also know that and the fact that he was there just happened to be unfortunate.
She is going to do what she wants to do, unfortunately, and no amount of guilt, shame, pressure or anything else is going to change that. You just need to focus on yourself and getting through this as best you can.
Acc
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015