No, those numbers don't exist. Even if they did, I wouldn't base this kind of decision on an arbitrary percentage. But that doesn't help you any.
If you are nervous about it, I would call the Sheriff and ask what he recommends you do. Be polite, and say that you want to be sure a used gun you are looking at wasn't stolen in a previous life.
They may have a way to run the numbers and check for you. I would mention this to your friend, and make sure that he knows he might lose the gun if it comes up hot. Better yet, if he wants to make this sale, have him do it. It would be better for him if the gun was stolen that he initiated the contact.
I haven't dealt with a stolen gun, but at one point I did unknowingly buy a stolen motorcycle. When I realized that this was the case, I was afraid that I would get nailed for having it. The first thing I did was get an attorney, and then called the police and requested an auto-theft specialist come to my house and take a look at my motorcycle. I told them at every turn that if something was fishy, I wanted to simply do the right thing and be sure it got back to it's rightful owner. It turned out that it was stolen, and since I initiated the contact with the police, I was never treated as a suspect.
I don't advocate that that course of action is always the correct one, and you should consult an attorney first (which I did).
In summary, I think you should do one of the following:
A) Have your friend contact the Sheriff. In this situation, he may lose the gun, but it's much better for him if he initiates this process.
B) Buy the gun, then immediately have the Sheriff check the S/N. In this scenario, you may lose the gun, and your friend will have to answer some questions.
C) Don't buy this gun, and find another one at a reputable dealer. If you do buy something stolen from an FLL dealer, at least there is a trail of official documentation leading away from you.