Can you get a more detailed description of this gun? Is there an actual date on the lockplate? It sounds like your friend could have a Model 1816 (also known as Model 1822)Springfield pattern contract musket. Eli, Jr., after his father's death was too young to take over the company and his uncles, P. & E.W. Blake were named as trustees and ran the Co. till he was old enough. After he took over, he held several contracts with the U.S. government. He built, along with the Whitney revolvers, many thousands of muskets including 15,001 Model 1861 Rifle Muskets. (These weren't exactly built according to U.S. specs.) If your friend's musket is flintlock however, it is probably the 1816 model from a contract let in the '40's. Specimens of this model that are still flint are fairly rare as most were converted to percussion in the 1840's and 50's. Many were converted as late as the Civil War. Find out if the barrel is held on by bands and whether there is a lack of a comb where the buttstock and the wrist join, as well as a date stamped on the lock. New Haven should be stamped on the lock behind the cock reading to the rear. Sounds like he has made a pretty good find.
And as Arilar said, the Whitney factory was in New Haven, Connecticut and continued to build rifles into at least the 1870's for the government. Unfortunately, I don't have a lot of info on this company handy.
:m2c: :thumbsup: