I agree with TAL, if you decide to confront, remaining calm is the key. When H finally admitted it to me, I asked questions, he answered, and I didn't cry once, didn't raise my voice. I thanked him for his 'overdue honesty', lol.
Can you imagine W's face after a knock on that door? Oh wow!!! Did you know the OM? My H's A was with a friend of mine from our playgroups. Lovely.
Do you want W to move out? I am so sorry you are a year into this marriage and this has happened.