Yes, I have two of these. One is a .40-65 and the other is the Marlin equivalent, the .4060 Marlin. (They are interchangeable cartridges)
The .40-65 is basically a necked down .45-70 case. It was introduced in 1887 for the 1886 Winchester. Basically a copy of Marlin's earlier .40-60, it uses bullets in the 260-320 grain weight. Muzzle velocites are in the 1400-1700 fps range, with muzzle energy in the 1160-1700 range.
Most powders that work well in the .45-70, also work well in the .40-65, but of course at different load ranges. 5744 and 3031 work well, as of course does good old blackpowder. Bullets are usually in the .406-.408" size, but your friend will need to slug the bore for the correct size bullet.
Hope this helps.
Ballard, the great American Rifles!