But essentially, somebody else sent her a package with something illegal in it that she didn't ask for. The police took her passport for a few months and searched her house. After a few months, she got her passport returned to her, she left Japan temporarily, but when she came back, they arrested her "to ensure [she] wouldn't flee while they finished the investigation".
She also mentioned it was "the most normal type of thing you can think of"; it might have been something like pseudoephedrine/Sudafed. That's a common over-the-counter drug in other countries but it's very illegal here in Japan (unless it's under 10%, or you buy it from Japan)!
Edit: Importing pseudoephedrine above 10% concentrations is illegal, but you can legally buy some higher concentrations over-the-counter while in Japan.