Triggers suck!
In most cases, the best way to deal with triggers is to disarm them. This requires a bit of introspection. I would spend however much time it took to get to the root of the trigger, figure out why whatever it was that was triggering me, and then decide (as well as I could) whether or not that particular issue was really worth all of that time and energy.
Triggers are usually about past events and not things currently happening. It feels real, immediate, and often overwhelming, but it's not real.
Your WW put her bestie in a very, very difficult situation. Whatever failings this friend has is her problem, not yours. Did she make the best choices? Probably not. However, in those types of situations, there are no good choices. It's all bad. And for what it's worth, I doubt this friend could have talked your WW out of her affair. You can't reason a person out of something they didn't reason themselves into. In the end, even if prodded, she told you the truth, even if she suspected doing so would end the friendship.
I'd say the same is somewhat true with you MIL. She couldn't talk your WW out of the affair, either. As for her silence... well, parents, you know. Our kids are always (usually) our first priority.
This friend's house is just a place. It has only the significance you give to it.
It's great that your WW respects your wishes and has limited contact with her friend. Infidelity has consequences, no doubt about that. However, as I've learned from reading from plenty of wayward spouses, the price of reconciliation cannot be infinite.
Married 2005
D-Day April, 2015
Divorced May, 2022
"The Universe is not short on wake-up calls. We're just quick to hit the snooze button." -Brene Brown