I was still living at home when my SO (we do not get along anymore) met my mother after a few dates. My mom has BPD, and when older women have it and it's untreated it's REALLY vicious and narcissistic and crazy. She was absolutely obsessed with every detail of my appearance. We had to walk part of the way and it was rainy out so my straightened hair got damp, and she was absolutely psychotic about how my hair looked, so I explained to him as we walked that she would be staring at my hair and extremely fixated on it, and that she would probably be curt and rude to him, which she was, but because I had to live there still I asked him to please just be peaceful and we would get out of there quickly. Well she stared at me with her psychotic violent eyes and talked endlessly about my "ruined" hair, and as predicted was snippy and dismissive to him, so being his oppositional obnoxious self he told her he was a communist to piss her off (this was the 90s and it was a very different outlook on communists then). This made my life hell for many weeks.
When I met his mother, who is a different flavour of abusive, I was not warned at all she was like that, and she gave me a highly unwelcoming look when I came in and he wandered off to fix her computer which she had demanded he come over and fix. I made feeble conversation attempts and talked about how I had been dress shopping that day for my friend's wedding and had gotten a good deal on a dress, I was the maid of honour, and she gave me a terrible look and stated that SHE only shopped second hand. I think I paid 40 dollars for the dress so it's not like I wasn't economical. That pretty much put an end to me trying to talk, and I just sat there miserably. I don't think he or his family ever really had a good picture until I came along and she started treating me poorly what a subtly abusive person she is, but she pits her kids against each other, says provocative things to upset them,called me by the wrong name on purpose since the day I met her despite being corrected several times, and does all sorts of other weird manipulative shit for attention, like pretending she's very poor (she wasn't and had more money than any of us) so giving us only secondhand Christmas gifts that she had taken for free from the Catholic charity shop she volunteered at, and wouldn't even put them in gift boxes but instead in produce boxes she took from the grocery store, full of shit nobody wanted and that we also just donated somewhere else, but also had a thing for giving you really insulting garbage gifts, like giving me her very used personal bathrobe that was positively threadbare, or waking the public golf course near her house and picking up discarded and chipped and broken golf tees and putting them in a Ziploc to give to her son in law who golfs. Or when she gave her only grandchild an old vitamin bottle filled with dish soap and a bubble wand she probably found on the ground at the park. Can't even spend one whole dollar on her only grandchild. She never worked a day in her life and got a very large alimony check every month for that time period, like 2-3K per month, did nothing for her kids but give them used crap they didn't want and threw out, and lived in this weird attention getting way, like if you went to her house for a meal and were trying to clear the table and asked her where the garbage was, and she'd tell you she didn't have a garbage can and to take it across the street to the public garbage can. Like deliberately narcissistically attention getting pseudoeccentricity. All sorts of shit like that. She's still like that even though we are long estranged for her shit behaviour to us, and I think my SO was relieved to have a reason to get away from her, and nobody in his family really has the ability to put into words all the nasty things she does or how it hurts them. So I understand why he didn't tell me because it's so hard to describe, but he also didn't really ever stand up for me, and that should have been my biggest red flag. She is just an awful person.
TL;DR, whatever it is warn them so they don't get hit in the face with it ever no matter how little you interact.