That's true. She could have dementia and not know what's going on. If it were me, I'd be tempted to walk over to her with my pants/underwear down and either jack off onto her face or have her hold her hands out to catch it.
I think his Nan/Granny gets off on it as clearly her grandson. As if didnt realise as ive seen other videos of the same guy masturbating in front of her and see glances over regularly. I bet she uses it as her wank bank, whilst fingering her clit and imagining her grandson emptying that spunk on her.