I almost hate to say it but in fairness to the dealer it is "possible" for a miss fueling event to cause major problems with the CP3 and other systems (on 2007 and later vehicles that includes some of the emissions systems). The chance of CP3 and other damage seems to be related to some combination of the amount of gasoline added vs the amount of diesel already in the tank as that interacts with how long the vehicle is driven after adding gasoline. Examples for a 20 gallon tank: 1) 1-2 gallons of gasoline the engine may run ok and damage is unlikely; 2) 10 gallons of gasoline the engine may run and if so run like cr$p and if run long damage, due to lack of diesel lubrication in the CP3, is somewhat likely; 3) 18 gallons of gasoline the engine probably won't run long enough to exit the station and damage is unlikely.
Hence while damage is "possible" it is not necessarily "probable".
This is where the dealer failed the normal/common sense procedure test either due to a subconscious

or conscious

reflex desire to throw expensive parts at the "possible" problem. That's just plain silly or stupid. In this situation the normal/common sense procedure is identical regardless of whether the CP3 is or is not damaged. Hey stupid drain and flush the fuel tank and lines as that HAS to be done and is the only sane first step to addressing the problem. Once that's done the vehicle will either crank and run or it won't. If it cranks and runs it might?? be worth pressure and flow testing the CP3 and injectors just in case. If it doesn't crank and run then and only then should CP3 and injector be investigated.
I've never done the miss fueling at a filling station but at age ca. 8 when my Dad was home for lunch I decided to "help" him by using a garden hose, hey it "looked" like the filling station hose, to fill his 1955 Chevy BelAir fuel tank with water. Vehicle ran like 10 seconds but once drained and flushed it was fine, not sure my butt was though.