Is she in the glass??
No. It means she drinks water from a glass that is not close to her, nor the person who's talking. She doesn't drink water from this glass, she drinks from that glass.
Why naquele and not nesse?
"Nesse" is when the glass is near, close to her. "Naquele", is used when the object is far from the person. The same in English, this and that
The girl drinks from that glass
Could it be " a menina bebe suco desse copo"???
I think it is much more acceptable...
I think that would mean that she drinks FROM that glass
It could be, "daquele" not "desse", same for "nesse" and "naquele". Different words and meanings. One is when the object is near and the other when it's far from you.
"The girl drinks the juice in that glass" sounds better to me.