With gluten if a friend was in the habit of complaining mightily that they HATE their enforced gluten free diet then, no, I would not stuff my face with bread in front of them. Because rubbing people's nose is their misfortunes is rude (I have no need to consume bread, you're medical needs may differ). It would depend of course on the friend. If I was visiting their space I would certainly not expect them to provide glutenous products.

It would depend of course on the friend.

Alcohol (and other drugs) are different because they affect social interactions though; being the only sober person in a room of drunk/stoned/high people is kinda terrible, especially if you want to partake and can't.