Quite simply, if it's a .45-70, it's not Civil War.
The Sharps rifles & carbines used in the War were not cartridge-firing rifles. The .45-70 Sharps came along later.
If it's truly CW, it'll be percussion. The .45-70 wasn't even in existence yet.
The Model 1869 was the first Sharps to use that caliber, so technically if your friend wants to consider that "Civil War Era", it's up to you how accurate you'd accept that statement to be.
From there, have you looked at any maker's stamps on the barrel or lock plate? Does it have markings that clearly identify it as a Sharps, or does it have markings that indicate it's Italian origin or from Shiloh Sharps?
Denis