I just think you need to be cautious. The OP's S will in the end be more worried about themselves and their own M than they are about you and yours. They will be more willing to believe their own S - esp if they think it will lead to reconciliation and a continued future with their S if that's what they want. While you are useful to them they will be your allie but basically they don't owe you anything just because their S is cheating with yours.

My H's OW's S didn't give a shi!t about me and helping me. Once that was obvious I just used him to feed info to OW that I wanted her to get.

Saffie

Saffie
me 46
H 46
M in 1986
D20,D18,S16,D13
H's A 01/05 to 07/06
H recommitted to M 07/06
renewed vows 09/06
Going from strength to strength