I would also start going to a marriage counsellor, get her parents involved and your church if you can. A huge myth married couples in trouble have is they think that BOTH spouses need to be present in order for MC to work or take place. MOST counsellors will see individuals with a wayward spouse and can help tremendously.
If you start going to MC, after a few sessions, invite her along in front of her parents and church members.
Ask her parents to throw this line at her :
"If you love your child at all you will at least see a marriage counsellor before you do something like this."
My guess is you two never went to counselling at all.
Its time to play the MC card.
Note, don't pressure her into MC directly. Just start going and let her parents and church pressure her. YOU just stand back and play the mature adult the whole time while the community pressures her to grow up.
As long as you stay cool, and have the support of your community, she will have a huge battle on her hands and inviting her to MC casually will just put the nail on her coffin of immaturity - assuming she refuses, which is likley.
Don't expect her to say yes, the ploy is for her to see YOU as mature and the OM as an interloper. You calmly and compassionately attending marriage counselling and church while she runs around with OM. This will eventually start to eat away at her conscience and plant a seed of doubt in her mind about OM.
