Likely a model 1881, about the first successful repeating rifle to handle the 45/70. I believe the 81 was the only top eject gun marlin made, so if it's an open top that's what your friend has.
If it's in good enough shape to shoot I would definitely stick with trapdoor safe loads. While factory stuff would likely be safe I would probably cook up some mild loads using cast bullets and Trailboss.
