There is no rule in the book forbidding the coffin flag from flying.
Most Vets will say to respect the flag by display only, to honor the one who gave all.
I do not have a coffin flag that I know of. But I have over 40 US flags, a lot of 9 1/2 footers.
I would buy or make a flag case, and start a grouping of the Vet, if he was a friend of mine.
I lost several friends in Nam. Have thought of this often, but would want the original flag.