Since W is living at home and openly engaging with OM, you MUST set boundaries and enforce them.
I got angry one day and told Ms. Wonka in no uncertain terms that the OW wasn't permitted to come near or in the house ever after I learned the extent of the A. Ms. Wonka heeded my boundary and met with the OW outside of the house. She knew by my tone and demeanor that I meant business. I didn't have to enforce it because she never crossed it because she CLEARLY understood it and frankly was scared to even test it.
No letter or email. You might want to compose a few and try out by yourself to ensure that you're clear and firm before actually telling W in person from a place of strength and calmness.
Do a few practice runs in front of the mirror when nobody is at home.
