Some people are here to throw barbs at others and receive them in kind. Some have a penchant for mocking others, even if under the guise of sarcasm. But then we have some people who don't care for that, who are simply here to share good things and aren't into verbal jabbing.
Ruaidhri's comment did have a winky face following it, but it's still an insult to someone who has made it clear that she doesn't play that way. A grandparent is a revered family member and strangers making jokes like that can't help but be seen as making a callous attempt at stirring up trouble for a brief thrill. It was out of place, especially given the history involved.
I don't know Ruaidhri at all, but perhaps it wasn't his intention to be callous. In any event an apology would be the right thing to do.