If the CAA had the time and inclination to do a full audit I wouldn't have thought that it would be all that difficult. Just think of the paper trail that is maintained at a FTO. From booking sheets and hobbs meter readings, prop logs, engine logs etc it isn't hard to marry up the total time flown from the FTO records to a pilots log book and note any discrepancies. Somewhere in LASORS or the FAA equivalent there will no doubt be rules about how long these records must be maintained which will invariably be a number of years at a minimum. If your friend logged landings at controlled airports then it would be relatively simple to obtain a list of tail numbers on the given days and check back to the logbook again. Pretty simple stuff really that a first year auditor at any of the Big 4 could do in a few hours.
Also don't forget that each time your logbook is submitted to the CAA that a copy is taken and whilst they may sign off your licence there and then there is no reason whatsoever to stop them from auditing the logbook at a later stage. I would imagine that there would be "understandings" between the different licensing agencies across the world to obtain information if they needed it as well.
Although there are anecdotes recited at every FTO that the CAA is weak on checking logbooks I don't believe this for a minute and pure logic would dictate that they regularly audit a sample of logbooks without any of us being any the wiser.