invite her friends to her place
And your friends
The correct way to say it is "Elle y est depuis quand" as "y" replaces the location in this context. "Elle l'est depuis quand" would mean "Since when is she it?", which doesn't make sense in this context.
Mon chez means my home (it should be - mon chez moi)
ça va chez vous ? = how are you, you'all?
home for example chez moi - my house
"Chez vous" means "at your place".
Chez François means "At Francis' place"
chez Magnot means 'at Magnot's (place)' in French.
If you mean 'elle' as in French, it is hi.
"we phone friends" "we are calling some friends"
the friends' house, the mates' house
I talk talk to my friends