To me it depends where she met this person. Hanging around the train station, probably not. A new mate from school? of course!
The done thing in my social world, in Australia
Is to phone the parents first & check out the who, what & where's. On the day/night of the party a parent will walk the child to the door & meet the parents. (much to the mortification of the child). haha. But at least they see every other kid going though the same thing.
My eldest 14YO, has just started going to parties with boys & girls. He goes to an all boys school so we didn't know the parents of the girls at all. So we make an effort to talk with their parents at social things on drop off & pick ups, like from the movies etc. I would never have dropped in on them prior to a party though!! I actually don't see the point.
We live on a gorgeous safe street where lots of the kids play together & the parents socialise when they want. We leave our garages up when home so kids know they can come over to play or that ours are allowed out. I might pop over to a neighbours house for a quick chat or if I need to borrow something and visa versa. However we all understand that this is our home & we don't make concessions for each other like feeling like we need to tidy up or put on something over our PJ's if it's still early.
That said, I would never drop in on a friend or family. I'd always call first. No one drops in on us either.