Musculata for 5th century?

[Flavivs Aetivs]

Of course there's the late 4th century Amazonomachy Mosaic which shows a Cavalryman with a Koblenz Helmet and a Bronze or Gold Sheathed Muscle Cuirass:



I recommend he saves up a good bit of money and orders from Jeffrey Hildebrandt of Royal Oak Armory in Canada. He does his research and can make your friend a good Late Roman muscle cuirass. He made some of Comitatus' gear I know.