My girlfriend slapped my mom

Last week, my girlfriend and my mom got into an argument. It started over something stupid—my mom made a comment about how “girls these days don’t know how to run a home” and my girlfriend took it personally. Words were exchanged. Voices got louder. I tried to calm things down, but I froze. Then suddenly, my girlfriend slapped my mom. Everyone went silent. My mom didn’t even yell. She just looked shocked and walked into her room. And I stood there… doing nothing. Afterwards, my girlfriend cried and said she lost control. She said my mom has always been passive-aggressive and that this was “years of frustration coming out.” She begged me not to leave her. My mom hasn’t talked to me properly since. She says she’s fine, but I know she’s hurt, not just by the slap, but by the fact that I didn’t defend her.