Dealing with Alcoholic in-law. Advice?

Hey guys,

So ever since my boyfriend and I started dating, I've known that his mom was an alcoholic. As much as it made me uncomfortable, I've just tried to stay out of it and keep my mouth shut to her. However things have changed. I posted a pic of my boyfriend on Facebook and said "Happy birthday to this loser. Happy 29, kiddo! 😘". Well she saw it today and was pissed. She was obviously drunk but messaged me telling me it was verbally abusive and how she wants the photo down. I didnt reply back but I told my BF and he's pretty pissed about it and is going to talk to her once she sobers up tomorrow.

What's the best way to deal with alcoholic in-laws? This honestly made me just feel so uncomfortable around her now.

Advice?