I had to say something or she would just continue to not check his homework which will affect him.
Does she check his homework now? My guess is, she's going to do what she's going to do.
I used to tell my H things about D's in the spirit of "how do you think we should handle..." At first he responded some. Then at one point he told me he felt like I wanted him to solve everything "right now".
They may see what's happening and just not have the wherewithall to really deal with it.
Would she respond any better (do you think) if the note was given to her? If you've tried and it isn't any better that way, I'm at a loss. You can't make her check his homework. It sounds like she see's your "lecturing" as a controlling thing.
I get it. It's about your son and his academics, this should be seperate from anything between you and her. It isn't always and esp early on.
I have a D that has been in counseling for over a year for some pretty serious stuff. H hasn't asked me or her C about any of it in over 8 months. He does talk to D about it. Given the issues, I am surprised he hasn't at least talked to her C or Dr.
I can tell you that whenever I talked with him about things with D's, I could see the pain and the doors shut behind his eyes (imagine the doors at NORAD).
You kids are much younger and it's going to be a hard row to hoe if you can't at least parent in parallel.
Who knows why she interprets things as your holding on. Too often it feels like your damned if you do and damned if you don't. Have you considered talking with a child C about ways to help your son?