Alternatively, what if a believer, with many friends who believe, demands that everyone should say grace before they eat?
This is not hypothetical. It has happened to me on a number of occasions.

Has it really Steve?
I find that extraordinary.
I have been at many, many meals where grace has been said but I have never, ever known more than one person to say it.

All that has ever been expected of the others present is the normal consideration of not interrupting anyone talking, but never to say it themselves too.

Even a mumbled "Amen" is entirely voluntary and optional.