20 dives shouldn't be enough to take even the Rescue Course. Although I personally don't think it is up to a fellow student to be turning another diver in for falsely filling in a log book, we all have to accept the fact that some people may do this, and hope that the Instructor is experienced enough to pick up on a diver's inexperience. Let's be realistic, if this guy is signing off fake dives to get his log book showing twenty dives then it will be embarrasingly obvious to the rest of the class that this individual shouldn't be in the divemaster program. I think responsibility in this case falls on the shop owner and Instructor. In the unlikely scenario that this individual is a natural and is skilled enough to take the divemaster course he still has to pass all the criteria, while although not overly difficult to an experienced diver, will be very hard for an inexperienced diver. My advice is let this diver spend the money to get certified, and let's hope the instructor see's this misrepresentation for what it's worth and makes the divemaster course long and grueling for him.