Are you really sure you want to argue electronics with an engineering technologist who's twice your age?
What I am telling you is that the charging circuits now used for these batteries know when to switch from full charge to trickle charge --amps to microamps-- based upon the voltage of the battery. The maximum safe voltage for each type of battery (NI-CaD, NI-MH, etc.) is a known quantity. All one has to do is design the charging circuit to switch at about 95% of that voltage and you have achieved the desired combination of longest per-charge use time without degrading the overall lifetime of the battery.
Yes, these batteries are rated for 1000 to 5000 deep charges depending on chemistry and manufacturing quality. As you point out that is about 3 years of daily usage... BUT, if you would stay with the original question, the concern was not about running the laptop on batteries every day it was about overcharging the batters because he *DOES NOT* run it on batteries all the time... The correct answer is that when not using the batteries every day, overcharging is no longer a problem... which is what I've been saying.
It's grade 2 math... how long will his batteries last him if he only runs them down once every 2 weeks? Yep that's right... 21 years.
Now... WTF is your problem????