Wow that is a tough one. At 29 your S is a an adult and it is doubtful you can sway him one way or the other. However, I would continue to tell him (if the topic comes up) that she is his mother and she will always be his mother. That she raised him and loved him.

Of course, she believes you put him up to it. WW always rewrite history and blame everything on H. In her mind, anything that is or goes wrong in her life will some how be attributed to you. Don't get sucked into that trap when she starts spewing. Validate, validate, validate, and then stick to your boundaries.

Me 41
W 33
M 2013
Suspect A 11/15
Confirm A 1/16
She moved out 2/14/16
Stepson 13
Stepson 16