Typically they contact a number of motorcycle dealers to ask what the retail value of the bike would be. Sounds like in your friends case since he declared the bike worth only $6K, that is what they paid him (assuming of course the quote received were for a higher amont). A quick check at autotrader.ca would show people are trying to sell similar bikes for $,7,500 or more.
I'm not an expert at insurance policies but I'm guessing the paperwork might say something to the effect of "retail value of bike as determined by XXX Insurance company or declared value, whichever is less". I guess it all goes back to why your friend declared $6K if the bike is worth more? Was he/she hoping to save money by riding a less expensive bike?
Not sure what recourse they have. Good idea to read the policy fully and perhaps contact a lawyer. Even better, I believe they can ask the insurance company for justification on the value. My guess is they could in turn ask for proof of what they paid, and if they paid more could try to weasel out of paying for misrepresentation.
Good luck.