Talk to his parents.
There should be a way to figure out the value of the bike- just get some research done on the internet to get those numbers. Armed with this information, if he thinks the bike is a junker and not worth the $25, I'd hope his parents would be impressed enough with the presentation to change their minds. He should also impress upon them that it's unfair to charge more than what the bike is worth, and he is just trying to be a good, honest businessman.
If it were my kid, I'd let him sell it for $15 after a presentation like that.
I don't think it's a good idea to go sneaking around behind his parents' backs after they told him how they wanted him to sell the bike. It's better to confront them in a non-confrontational, intelligent manner, armed with the evidence on why he thinks the bike is worth $15 and not $25. However, if the bike is really worth $25, why not sell it for $25? If that's the case, try the local university and see if there are any college students interested in buying. College students usually need bikes, and they need them cheap.
I see you are a bit of a newbie from my home state. Welcome!
Good luck, and keep us informed on how it all unfolds.