I still say you can send your MIL one IF IF it's done as a way of thanking her for what she has meant to you (assuming this is authentic)

and say it in a way that shows you accept your w's decision. You just didn't want to leave your MILs life without ever telling her what she meant to you. Keep it brief, like under 1-2 paragraphs and wish them well

I think it's safe to do especially since you are NOT sending your w a card.

M: 57 H: 60
M: 35 yrs
S30,D28,D19
H off to Alaska 2006
Recon 7/07- 8/08
*2016*
X = "ALASKA 2.0"
GROUND HOG DAY
I File D 10/16
OW
DIV 2/26/2018
X marries OW 5/2016

= CLOSURE 4 ME
Embrace the Change