That is one of the cheaply made revolvers imported from Germany in the late 1950's and early 1960's until the Gun Control Act of 1968 banned them from importation. They were often called "Saturday Night Specials" and sold at the time for as little as $9; they probably weren't worth that. The frames were a zinc alloy and would not stand up to much use. Some had failures like loading gates blown off and barrels coming out, and broken hands and cylinder stops were common.
There were a couple of brand names, the main one being Rohm Gesselschaft (RG). I think that is an RG-10 but, as I said, there were several similar guns, and Rohm supplied parts to other companies. Many had the maker's name on the grips, some did not have it anywhere.
Naturally, with so many made, some worked OK and a few folks even tell of how they shot them for years with no problems. Sorry, but they are still junk, even if they are working junk.
If he wants to keep it as a remembrance of his grandfather, fine, but he needs to be aware that the value is near nothing and the gun is not worth spending money on even though some parts are available. No dealer will take it in trade and I know of no one who would pay money for it.
Jim