1) No need for theatrics. Just tell him simply, calmly, that you know he's still talking to OW and that you have filed for divorce.
2) I'd pay heed to your friend's concern about you telling your H alone. If she thinks he might get violent, she might be picking up on some vibe that you're too close to to notice. I'm not sure what the best way to do this is - somehow it seems worse to have another person around to witness his shame - maybe you could tell him in some public place? Or have your friend come over immediately after?
