DS(10) is normally quiet and quite polite. However, he can explode a bit if pushed.
He was at a friend's house last night and I picked him up at just after 8. The mum seemed a bit quiet and this morning she sent a text asking if we could talk so I called her.
Her version is that DS shouted and slammed a glass down on the table and it made everybody feel very uncomfortable. Obviously I asked what brought it on and she said it was because her mum, so his friend's grandma, had offered DS a sandwich!
So I spoke to DS. He started crying 
and said he did but he didn't really like any of them so kept saying no thank you, no thank you. But apparently the gran kept saying go on, have one, they are nice, I've been making these all afternoon. He said she was waving one in his face when he shouted 'no, I said no.' (I actually taught him that phrase a while back.)
So - discipline or not? I'm not sure how to deal with this. I hate rudeness but I hate people who won't say no for an answer as well.