I always assumed it was in her things from the nursing home. Bojack never looked through them until after she was gone, so he wouldn’t have seen/registered it (remember how he doesn’t notice Hollyhock’s envelope in Beatrice’s things until after both of them are gone?). She clearly remained obsessed with her weight until the day she died, so I don’t think it’s a stretch to believe that she could have been taking it in the nursing home, and while sneaking it into Hollyhock’s coffee.
I always assumed it was in her things from the nursing home. Bojack never looked through them until after she was gone, so he wouldn’t have seen/registered it (remember how he doesn’t notice Hollyhock’s envelope in Beatrice’s things until after both of them are gone?). She clearly remained obsessed with her weight until the day she died, so I don’t think it’s a stretch to believe that she could have been taking it in the nursing home, and while sneaking it into Hollyhock’s coffee.