AK103K said:
Just restamp the number. Its not illegal to remove the number if you restamp it. Its illegal to remove it for "unlawful" purposes.
Incorrect.
Federal law (and most state laws) prohibits removing or altering a serial number. It IS illegal to remove and restamp a serial number. The law says nothing about the purpose of removing a serial number -- the law says you can't do it. And, as has been pointed out, the same Federal law stipulates that whoever is in possession of the firearm when the alteration is discovered is legally presumed to be the dastard who committed the felonious act ... so this is not something to treat lightly.
However -- the original post said the "gangsta" filed the serial number off the slide. Federal law requires that the serial number be located on the receiver. There should be a number on the receiver, and if this is the case the fact the non-required number on the slide has been altered/obscured/removed "may" not be a violation. As an example, Argentinean Sistema 1911s have a serial number on the slide and the barrel as well as on the receiver. I don't think anyone has ever gotten in trouble for replacing a Sistema barrel.
I would suggest that your friend examine the pistol carefully and determine if there is, in fact, an unaltered serial number on the receiver. If so, he could then write to the BATFE and ask for clarification as to whether the serial number on the slide is required, or if it can legally be removed or obscured.
If there is no serial number on the receiver, your friend should have a lawyer contact the BATFE and explain the situation. The BATFE does have a procedure to stamp a replacement serial number on a firearm, but the work has to be done by a gunsmith approved by the BATFE and strict protocols have to be followed. My guess is the process would cost more than buying a new Mosquito and, worst case, he would be better off surrendering the gun if the number that was defaced is the only serial number on it.