I don't quite understand the grammar for drinking all of you that they could. And the meaning of the whole sentence is not quite clear to me either.
They treated you as if they were never going to see you again, drinking all of you that they could.
Update with more context:
Do you remember the night of my first high school party? You were there. My friends were intrigued by you. They treated you as if they were never going to see you again, drinking all of you that they could. I spent two hours that night helping my friends who had fallen completely. “I‟m so embarrassed,” they said as I held their hair back so that they could vomit. “I‟m sorry,” they said when I called taxies for them, walking them out and paying the driver in advance. “This won‟t happen again,” they said as they were sent to the hospital to have their stomachs pumped. Two 15-year-old girls slept in hospital beds that night thanks to you.