Parents abusive relationship

So ever since I was young my mom has always made comments about her hating my dad and we always thought she was being sarcastic because of my mom‘s personality type but within the last year she has confided in me a lot and it turns out my dad is actually a really shitty person towards her And my whole childhood my mom was just extremely unhappy, not being sarcastic she was serious. He’s cheated on her, gave her STDs, things I can’t even fathom saying to anyone. Recently she tried to leave him and she told me that he put a gun to his head and was grabbing her by her shirt not letting her leave the house. I know this isn’t my relationship but I am fearful for my mom. I don’t know what to do or how to help her and she says she can’t leave because there are a few things that she needs to get in order before she does, also the concern for my dad‘s actions is another thing. Does anyone have any advice? I feel helpless but I’m worried about my mom