Hey there. I can totally relate to this, so I thought I'd respond.

First off, I am really sorry for what you are going through. The thing is now that you've realized what the dynamics are between you and your mother, and sadly it's going to get worse when you break away - which is something you know you have to do.

As for how she feels about your boyfriend, she probably had her feelings about him prior to talking with her "friend", but now she feels she has validation for her complaints because this person doesn't like him.

The only thing you can do is stand up for yourself in some way. While you think you are by saying "you don't know him like I do, Mom" you know she's just tuning you out.

I know it sucks, but you have to find it in you to stand up and act. Otherwise she will just keep poking at you, making you feel guilty about anything and everything beyond your control, and blah blah blah. Things might get worse when you do this, but in the long run, it's your happiness now that counts.
I wish you the best.

PS - I know because I've been there. I moved away a long time ago and can't look back. It's too much.