I've been yes/no/yes/no through the thread as you've been posting.
I'd be very worried that's she ok with family, but no one else as I'd be scared she's been upset/hurt/abused by a non family member. Also with her being ok if your friend brings her DD but not as an adult on their own, I'd be concerned that she felt 'safer' if there was another girl her age there. Not that 'this' friend of yours has done anything, just an adult on their own.
Kids can be manipulative & controlling, there's no doubt 8 yo's can play you like a pack of cards.
However, they can also be scared & unable to explain it. My cousins used to babysit us, when I was your DD's age I started to get very upset if the eldest one was babysitting, but not the middle one (both boys). No one listened to me, they just told me not to be silly & that I loved him so stop being daft. Thank god he wasn't abusing me because I wouldn't have been able to 'tell'. The reason I didn't like him babysitting all of a sudden was that he was bringing his gf (my parents knew that) and I liked her, but not when they were babysitting - simply because they were so wound up in each other & kissing (just kissing) I was scared they weren't looking after us properly & keeping us safe, especially my younger siblings. We. Were. In. Bed! As an adult I can see why it seemed daft to the adults, as a child I was just shit scared in a way I couldn't explain.
When did your Ex move out?
For tonight, does your DD have another friend she could invite for a sleepover?
Then tomorow, you need to talk. You need to get this sorted out. Engineer a way to talk while driving/cooking/walking etc so it's not so intense & try to get her to just chat without it seeming like an interrogation. Or is there someone she's more likely to,open up to like a grandparent? That's no refection on you, but sometimes it's easier if it's someone whose feelings they can't hurt & isn't so close to the situation.
Go tonight & have a good time. She will be fine the minute you close the door. 🍷🍷