Many years ago, I turned a girlfriend on to LSD. She loved it, and wanted more the next day. I told her that she ought to wait a few days, for the tolerance to fade. But she was adamant.
So we did maybe 2-3 doses, and she was still unsatisfied with the peak. And -- dumbass that I was -- I suggested that we smoke some marijuana.
That was a serious mistake. Because our short-term memory dropped to 30 seconds or so. I was cool with that, staring at the patterns in everything. But she was not. So I spent perhaps 3-4 hours reminding her that she was just wasted on acid and dope, and that she'd be fine tomorrow.
And she was. Except that she was no longer my girlfriend.
But whatever. Maybe you could have just reminded your grandfather that his short-term memory was hosed. And then assured him that you'd be back next week.