If they are confused at the difference between Static memory (HDD) and dynamic memory (RAM) then it is possible that they have never been told.
Now you're confused. a Hard drive is not "memory" I'm pretty sure memory is always solid state.... and also what about SRAM? (Static-RAM) it's used for most CPU caches because of it's speed but you would hardly compare it to a HDD.
Make sure he is also emptying the recycle bin.
This might be the case. I often forget to do this and wonder myself, actually. (doesn't take long of course to realize that the recycle bin contains items). Of course this is assuming we are talking about hard disk space, not memory.
PAHAHAHA... You're going to kill me... I am 16 and I know the difference between them (I think I was 12 when i found out )
Seriously... it's not funny.