I'm guessing you guys that are recommending he call a lawyer have either never dealt with lawyers or are lawyers yourselves. A lawyer's fees would exceed the value of that gun real quick.
I would call Ruger and explain the situation. If that doesn't fix the problem, I would destroy it before I paid a lawyer.
Or maybe find somebody that could stamp the serial number back in the gun.
I'm a lawyer and I have a real hate/love relationship with people who take bogus legal advice and then **tch at me when they need me to fix their self-inflicted problems.
Yup, cheap by you last week can earn me thousands of dollars later on.
Cost of the firearm ain't squat compared to what you could (yeah, not guaranteed) wind up paying to defend against whatever charges could come around later. Depending on the state, it may very well be illegal to destroy a handgun frame without appropriately reporting it.
The first step of reporting the crime against the property is always recommended, and *might* be enough in this circumstance. However, this is an opinion and not legal advice because I haven't seen the s/n, I haven't interviewed the crime victim to ascertain the facts, I haven't researched the local and federal law (but know that, IIRC, it's only gunsmiths, manufacturers, and others authorized under the federal scheme to take possession of other peoples' firearms overnight for repair, etc., who can change s/n locations, re-stamp, etc.), and nobody's paying me for my knowledge and expertise in this matter.
Sometimes attorneys sell not from fear, but for peace of mind and *reliability* of the answer obtained.
A $50-$150 consult may be all that's needed, but not with just any lawyer. Find someone who knows firearms law from the criminal defense perspective.
Calling ATF often gets very reliable advice, but they are *not* your friends, and anything you say can and will be used against you.