My dad cheated on my mom. She's absolutely devastated and i don't know how to help her.

My parents have been married for 30+ years but it wasn't the best marriage. My dad has a really bad habit of drinking and it usually affects his way of feeling and thinking. He can be extremely happy one second and then really angry a few moments later. It's a complete mess. My mom had to deal with that for many years. This relationship has been draining her forever but she still dearly loves dad. This morning, my dad told my mom that he's been seeing somebody besides her and it destroyed her completely. She got drunk after finding out and has been sleeping all day. I'm 14 yo and i have no clue on how to comfort and support my mom. I told my older sister about this but she lives in a different country and getting in contact is difficult because of the time difference. Please give me advice on how to help my mom and deal with the situation

tl;dr: My mom needs support after being cheated on. How do i comfort and help her?