Copyright law is interesting because it's at the boundary of criminal and civil law. But in this case, the criminal side can be safely ignored. Criminal prosecution has a higher standard of evidence, and this case is murky enough that there is insufficient public interest for the State to press charges.
This leaves us with the civil side, as apsillers already noted. The original author can sue for two main reasons: to stop an infringement, and to claim damages.
The first demand (stop infringement) is something you don't need to worry about, because that would just put you back in the default situation.
The damages claim is therefore the only one that deserves further analysis. Again, aspillers had a good comment: estoppel and laches may be available as defenses. But that is further down the line. The original author first needs to explain in the suit what action of yours was not permitted. That claim would be, in legal terms, the creation of a derivative work. Technically, that takes the shape of disassembled C code, but legally the particular shape of the derivative work does not matter.
Now the first defense of yours will be that regardless of the GPL version, you have the permission to make derivative works. Estoppel and laches are additional defenses - if you had permission anyway, there was no copyright violation. But this is a legal technicality: was the GPL offer direct permission or merely a promise thereof?
Either way, the burden of proof is on the claimant, and courts tend to take a rather dim view of claimants that caused problems for themselves. It would have been trivial not to mark the program as GPL.
