I think you sound like a great parent Frig your boy sounds lovely.
As you'll know, 5 is no indication of personality, so I dare say this little girl won't always be so unassertive, or biddable I dare say.
My ds (now 10) is fairly biddable, always has been. I did genuinely worry about his empathy levels at one stage, but this was more about my concern he'd turn out like his awful father. He's adorable, caring and intuitive. :) AND he's boisterous and loud when he's with his mates. I can still give him that look tho...
Your friend, I reckon is in for a shock..
I will say though, when I've been in a car with ds and his mates and they are all making silly noises and stuff, I can barely cope! It was a horrific shock to the system. I looked at the parent driving and she barely registered.. So I assumed it was me being a parent of one, and got over myself.
The drive home was the same, I just sat on my hands so as not to hit the window button and jump out... 😂
When you have one child the noise level is one thing, add in another child and the noise level is not doubled, it's doubled and then some. Add another child and it's off the charts :)
I can't bear screeching, it's an immediate no for me. My neighbours DD does it in the garden sometimes, I have to close the windows/doors... IME boys roar and make silly noises, screeching does seem to be a thing that girls mostly do.
I also lost a friend last year, probably due to her son. He is by her own definition, (and everyone else's) a nightmare. Attention demanding, won't sleep until thebearlyvhours because he won't give adults their time without him. He's rude, smart mouthed and knows exactly what he's doing. He also knows that his family won't do a thing.
I was expected to look after him during my holiday, when it was just me, ds and him he was an angel. When his family was around he was awful. I did have to say to him to not shout What the hell bro repeatedly and at top volume on the beach. Ds and I were the only English people on the beach and it's not a thing I'd like my ds to be shouting, so I told them both that it had to stop. I also told him that if ds didn't want his name put on a game online he didn't have to have the boy do it.
He answered me back really rudely. I did tell him off. There was no way I'd put up with that.
No adult ever constructively tells him off. His mother in the end was more interested in her new boyf than her kids, and his needs trumped even our visit. He was vile and I suspect extremely dangerous to have around kids, but that's another story.
If I were you I'd have suggest she sees you alone, or drops off her dd. And tell her why. It's not on for her to over parent your child, and actually he's not doing anything wrong.