Sowhathappens, this is a female friend of yours and your DH? She's married with kids.
Time to get ghetto on that OW!
You know the couple? you know where they live, GO ROUND THERE, tonight. Leave DC with DH, then you know he can't go anywhere. For once he'll be right where he's supposed to be.
Take a taxi and make it wait for you if you have to. Better yet, if you have mutual friends of theirs and yours, get them to take you round there, so they can see what kind of whore she is.
If you can take your DH phone with you to prove to her DH what you are talking about, do it. Assuming that her DH doesn't know already about it....
Sit on their sofa and spill it all, the PG, the fact that he's being an arse, and the fact that despite being a so-called friend, she is shaging him and even knowing that you are PG she is not leaving him alone.
Tell them it stops right here, right now. Tell them this is your life, your family she is literally fucking with, and you are not going to take it. The friendship is OVER as of this very second. FIGHT WOMAN FIGHT!
Then, that job done, go back to DH and tell him it's over, and that if he goes near her again, not to bother coming back. MEAN IT.
He is on his very last chance, one slip, one email, one text, one call and he is out. You would be better on your own than with someone who has that little respect for his family.
Huge hugs for you, now go to war, fight for your marriage!