It really does suck finding out that your H is spoiling OW. I remember how sick I felt when I found out that W had been splurging on OM (even though she cant afford it). It really sucks, because she never splurged on me, as I always did for her.
Also, in 17 years together, I can count on two hands the times she actually cooked dinner for our family (as I am the culinary king in our household). I have found out that she regularly cooks for OM at his place (might not be the most elaborate meals, but its the thought).
I think they need to validate everything they have done, and their feelings for OP, that they accentuate the thoughfullness and generosity (in the fear that it may not work, or OP may leave them).
Anyway, that is my threepence worth. All my best, AndyV