I'd check their profile to see if they are just randomly searching for coin tracking numbers and logging them. If they are, I'd report them. If this was an isolated instance, then it is just possible that they transposed a number of an actual coin they saw, when they logged it. I have transposed many GCxxxx numbers when logging caches and have done so on a couple of coins. I noticed it was the wrong cache or coin before actually logging, but if one isn't paying attention...
Again, I'd give them the benefit of the doubt and check their profile before reporting.
-HiDude_98