yes, a Federal Eagle Infantry "I" Button, i cant read the backmark
tell your buddy NOT to scrub it or clean it yet.
wait someone will say whats the best clean, to read backmark
i think there are a couple of fed buttons that are hard to find
Looks like a squished 2 Piece that has some serious crud on the back.
I'm betting it's CW period by looking at the design. Spanish American period eagle buttons had a raised shield and the eagle legs are typically longer. Back mark would verify for sure.
Can you give us an exact measurement of the diameter? If it measures 7/8" then it would be an officers button and that size was produced from 1851 until 1902. If it is smaller than that it is most likely an enlisted man's button and was produced until 1854. In 1854 the letter designation was removed from all enlisted men's buttons. It could still have seen service in the civil war because the letter buttons were not taken out of service and existing supplies were used until they ran out.