From personal experience--it was not helpful for me to do this. I didn't contact H's parents--they're a little too wacky--but rather a brother-in-law and sister-in-law, partly because SIL is a psychiatrist and they're the in-laws I've felt closest to. I even provided proof in the form of emails. Initially they were very supportive, I felt sure they would talk with H. But very shortly, H contacted them and told them god knows what--and now I have been cut off from even casual contact. They want no part of me. However, very supportive of H--altho I'm sure he has denied the A in spite of my proof.
So even though it seemed a sure thing and a well-considered plan, it pretty much blew up in my face. That's just my experience; others have had better results.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012