I'm not an expert. These are things I would personally check for.
Where did he get the revolver from? Any history on the revolver is good to have. Check the markings and see if they are correct and that the serial numbers match. The grip frame, grips(marked with pencil on the inside), trigger guard, the frame, the cylinder, the barrel wedge, and the barrel all should have serial numbers that hopefully match, and they should be int he same font. If they are not marked they may be replacements; the wedge is usually the culprit here. If all numbers match it adds to the value, if they don't it takes away. A lot of these are found in the white but that often lowers the value some. If there is any finish or patina on them it adds to the value. You can check for remnants of the original finish-there is usually case hardening left on the rammer where it pivots on the barrel, there is usually some case hardening left on the frame under the trigger guard, the trigger guards and grip frames were sometimes silver plated brass so you can find remnants of that on the undersides of them. Modifying the rear sight and front sights were fairly common for guys that actually used them a lot. If the modification is historic and wasn't just done by the guy you might buy it from, that can add value or at least not take value away. If its the original front sight it will usually be worn down. Look at the bore condition and condition of the screws. 99% Originals will not have a perfect bore. They will be worn and pitted to some degree. The screw heads on originals will usually be messed up or worn to some degree or another. Fakes often get the color right, but the screws and bore will be perfect. It sounds like the timing and lockup are fine if its been shot. If you plan on shooting it, which you should in my opinion, check the nipples on the cylinder. A lot are rusted in place and require drilling out. If its the original cylinder and you can get the nipples out, that is a huge plus if you want to shoot it. As far a price, they start around 1000 and go from there. As others have said check comparable revolvers and you might ask him what he wants for it. I've found lots of times if the owner knows it will go to a good home they are very reasonable.