OP,
I think your post is being extremely unhelpfully picked a part.
I honestly wouldn't be bothered my arse coming on here, asking for well meaning advice, from some posters who have little to do but look for offence, when clearly none was intended.
Living in the real world, as I do, I regularly describe my friends as girls.
I do not say I'm playing x sport with the women, I say girls.
I don't say I'm going out for dinner with the women, I say girls.
I have coffee with my girlfriends...horror of horrors and nobody has died.
In my extremely large circle, women describe women often as girls and none of us have died because of it.
The OP has described someone very young.
About early 20's...yes definitely a girl.
In my early 20's with an excellent professional career, I thought of myself as a young girl, I was super confident too.
With a big age gap between her and her mean shouty partner, and if she is a soft gentle soul, I can well imagine a "type" that could be bullied buy some Sargent Major prick.
OP, it sounds awful for that mother, who may now feel stuck, and those poor children.
Definitely call SS.
Tell them you do not wish to draw the nasty twat on you and do NOT wish to be identified.
Make it clear you are concerned for her.
You sound like a nice person OP👍