My mom just had something similar happen, in bringing up something personal as an attack. Mom likes to fling mud at Trump supporters on facebook, nothing mean spirited, mostly at people she knows, but one of them who she has known for over 20 years decided he was gonna publicly attack her using some personal stuff about her and my father that not everyone knows. It was one of those "Trump does <thing Trump actually does>" countered with "oh yeah well look at <thing mom did>, how can you point fingers???"
Mom says he apologized like 2 days later and I told her she needs to just tell the dude to fuck off and block him, but she's a good person so she half-apologized about her mudslinging.
Extra points for my dad having been dead for 9 years
Sounds like her friend is a vindictive asshole. Unable to counter an argument without resorting to personal attacks, she'd be better off without them in her life. It will happen again and maybe next time it won't end as civil.