Subterranean is correct.. although he's a bit off on the dates, which are actually 1875 to 1902. Your friend's button is what button-collectors call the Indian Wars Era eagle ("general service") button, but in actuality it dates past that era, through the Spanish-American War era, to 1902, when it was replaced by the Great Seal general-service button.
The backmark on your friend's button seems to be the very-late-1800s-through-early-1900s version of "Scovill Mfg. Co. / Waterbury Ct" backmark.
The photo below shows the three versions of US Army "eagle button." Their period of manufacture/issuance respectively (from left to right) are 1902-to-present, 1874-to-1902, and 1854-to-1874.