Need a bit to vent, but also would appreciate opinions on this. Sorry in advance for the lengthy post..

Backstory: After 29 years of marriage, my dad decided last January (25 years) to move out and begin the divorce process from my mom. She of course took it very hard and continues to even now. 

She refused to, and in the end did not attend my wedding last August bc she couldn't stand to be in the same space as my father (granted it was my 2 marriage, but still very hurtful to me and my hubby).

Flash forward to now, over a year and a half later-they're still not officially divorced bc my mom states she will "sign the papers in her own time" when she is at peace with it. She continues to spend time looking over his Facebook and I hear about EVERY comment he ever posts (which I keep telling her isn't good for her and she should unfriend, block and be done with that, but she won't). Every time i see her she asks about him and then proceeds to take over the conversation and talk non-stop about anything involving him. She sees a counselor and has since the initial split, but it doesn't seem to be helping much for her sanity.

Now the "I need opinions part":

My dad has had a few girlfriends since he moved out last year and this latest one he has been with for over 6 months-ish now. My mom has been aware of  each and always asks about them as well. This past week, she asked if i had seen "them" recently and me, not going to lie, told her my hubby and I were going to have dinner with them this weekend. My mom went on a tirade asking why I felt like I needed to invite "that whore" (aka my dads gf) and "encourage his relationship with her" and how I don't owe her anything since she's nothing in relation to me. She even went so far as to say i should UN-invite my dads gf from dinner. 

A night later my mom proceeded to write me a 2 page email saying how hurt she was, how unsupported she felt by me etc etc. I messaged her to apologize that she felt that way (I refuse to apologize for including my dads gf as I have no personal problems with her) and now, as usual after her and I fight, my mother isn't speaking with me. I just wanted other opinions/takes on this-am I in the right here or does my mother really have a reason to be upset with me over this. Dic