How do you know they are in contact? If she's blatantly doing in your face, you will have to make a hard decision. If you are snooping stop. You don't need to snoop anymore for now. You have all the proof you need.
File when you are ready to file. You can't control her, and if you're filing to try and get through to her it will only backfire. But if you're filing to truly heal and gain some space from the destruction, I understand that as well.
Stop letting your WW blame you. Next time she blames you, strongly and directly say, "it was your choice to cheat".
If she says, "you told the OM wife, and if you wouldn't of, nobody would know. You say, "it was your choice to cheat". Don't get in a back and forth, or blame game with her.
If she says, the whole town knows because of you. You say, "It was your CHOICE to cheat". Stay it clear, concise, and with confidence. If she keeps trying to get you to take the blame walk off, if you are on the phone, say, "I'm hanging up now".
Those words and actions, SHOWS her that her blaming won't be tolerated.
I'm sorry your son has to go through this, but you have to be his shield and armor thru all of this.
Onward and upward
M:36 W:36 T:11 M:10 S16, S12, S9, S4 BD:06/28/17 OM confirmed 07/20/17 Recon the M 10/29/17 Working hard:2gether
Onward and forward
This process is not a sprint it's a marathon! Patience, Patience, Patience.