With luck, it will be OK, but 3 weeks is kind of a long time without someone knowledgeable checking things (temps & water) to make sure.
And without some very real coaching, it's asking A LOT of your 80-yr old grandmother IF she has no experience or knowledge of snakes...what if she reaches in
(for the water bowl) & the snake bites & wraps her arm? It's not likely but far from impossible and she could be in real trouble. (I'm thinking more like a heart
attack than the snake actually trying to have her for dinner...) I wouldn't want that on my conscience...just not a good plan, especially since the snake will be
getting hungry during this time frame. And if you feed right before you leave, what if something makes it regurge? Would she be able to handle that? Again,
not likely, but I'd look for a more appropriate "snake sitter".