I am still firmly in the 'tell her' camp.
As Princess Diana famously said, there are 3 people in this relationship, but only 2 of them are knowingly choosing to be.
I understand you wanting to remain anonymous, OP.
So maybe print off the photos, give some dates when you know they've been together, and tell her the OW name.
That is evidence he can't easily explain away.
I would also tell her that the initial affair ended 5 years ago because he decided not to leave his wife.
Maybe the married couple do have an 'arrangement ' re discreet playing away, and the wife will just ignore your letter, and things will play out.
Maybe this time he'll leave and go to your friend.
Maybe they'll stay together, work on their marriage, make a go of it.
Whatever happens at least the wife can make decisions about her life based on full knowledge.
If it were me, I would tell her.