Yep, you know what? (and you can read this to grandma) So many people have an image of a rat and they think of vermin, sewers, the Plague and so on and so forth and in the horror movies they are made to look scary and threatening, and so forth. Media and myths are highly seductive to the public when they have no other information to go by.
Now, add to this equation a generational thing where people of a certain age group may have grown up in a time where rats were only seen as crop destroyers, "rats in the barn, eating the corn, oh nooo!" and that kind of thing and it is very easy for certain generations to think poorly of these amazing and very loving, clean and gentle animals.
My husband and I are both 45 years old, with no human kids, but we LOVE our two sweet baby rat boys. They are 120 percent our furry kids and we would do anything in the world for them. They give us soooo much love and fulfillment, every day. Now, if your grandma says, "Well, people that love rats are just weirdo punk type people", tell her that my husband and I both are college educated professionals from Seattle and yes, we are stable, secure and intelligent and not weirdos. We just love animals and there are a LOT more rat lovers out there nowadays...and they come from every walk of life.
You are right...rats are far more less apt to be nippy or bite than your average hamster. They are actually the least apt to bite or nip than any of the other small animal pets. But the poor guys STILL get a bum rap and get called names or cause panic out of sheer ignorance.
Rats are also exceptionally clean, quiet and very, very sweet little guys/girls. I always feel sorry for rats because they are the most sweet tempered and gentle of the small animals but other small pets like hamsters, that can have tempers or nip get all good PR
and the rats get the short end of the stick, yes? Your grandma probably DID have an episode with a wild rat, which can be very different! We always say on the forum that a domesticated pet rat compared to a wild rat is like comparing a poodle to a rabid hyena.
See if grandma might be willing to observe Rue a bit and see how sweet your boy is! I know he is not feeling well right now, but hopefully soon, and he will be up and about, feeling better. Tell your grandmother that my mother is 74 years old and she was "afraid" of rats when we adopted our rat boys and she ended up LOVING them so much...after she met them, petted them and I told her about the huge difference between wild and domestic rats...she fell so in love with them, that she always wanted to bring toys and things over for them and she always asked to pet and kiss them, once she found out how personable and wonderful they were. She even became our regular babysitter for them!