I will soon be visiting a friend who is a graduate student. She works in a communal office with around 50 other desks (which are mostly occupied by other grad students). Is it appropriate for me to study at an empty desk?
Think of it like visiting somebody else's house. Your friend is your host, but also shares her "house" with a bunch of roommates. She is the one who will know best what sort of etiquette is followed in her community.
Why not ask her, "Where would be a good place for me to study around here?" You might get sent to one of the desks, or to an empty meeting room, or the student lounge, or the coffee shop down the street. There's no way to know without knowing the lab culture, and your friend is the one in the best position to judge.