You said she sends horrible messages often and that you both went no contact from her horrible treatment. No contact means just that. Why would you or him message her just to make her feel better about him not messaging her for mothers day? She doesn't deserve it, and it will only make her think that her behaviour is okay. @Ash thats a bad idea because it will just reinforce her horrible behaviour, mil doesn't care about anyone but herself, so why would she care now about him prioritising his nuclear family? When mil acts like a real loving mother, then she can be messaged on mothers day, until then continue no contact and mute her messages.
It's normal for a narcissistic person as everything is about them at all times. I would advise your partner to cut her off completely and block her from all possible contact routes so that he can look after his own peace. My ex mother in law was like this and she never once thought that she was in the wrong, even when she put my car window through because her son hid her sellotape, as a joke. In my experience if she has a way in to you then she will use it to try to destroy your relationship
@Ash I asked him if he wanted to but doesn’t seem like he does. I think he’s more upset with her than I am but of course it’s his choice..but guarantee that’ll be my fault from her perspective though 😂