First, be sure you've got the right serial number - take it from the butt of the grip frame. With a short number and no letter prefix, it's possible to get it confused with "assembly numbers" that also appear on the gun.
Assuming that your friend's Model 40 does has a 4-digit SN starting with 6, it would be from the 1952 (SN 1) to 1971 (SN 30160) period. Interpolating would give a date of 1956-57, which is only an estimate, so the gun would appear to be from right around when S&W started assigning model numbers to its revolvers (1957). If it does have the model number stamped in the yoke cutout then it's from then or a bit later. The Model 40 was originally called the Centennial because it was introduced on S&W's 100th birthday as a company.
Because of the grip safety, which is an unusual feature as part of the hand ejector design and was not retained when the same basic gun was re-introduced some years later, Model 40s are somewhat more valuable than similar J-frames. In the condition you describe, I'd estimate it's somewhere in the $500 range, perhaps a bit more.