Jesus, what the fuck? My paternal grandmother died in '04, and the maternal one in '11......never at any point in my life would I have felt remotely comfortable doing this lol.
Even if he has a closer relationship with his grandma than I did mine...I can't imagine doing that.
That is the type of playing around I'd do with girls I've had non-platonic relationships in the past. Not saying there's anything wrong going on between him and his grandma, I obviously just don't share their sense of humour and it's weird as fuck to me.