One thing that the other answers have omitted is that, in modern operating systems, the entire hibernated contents of the RAM are not written back right away upon awakening. Enough is written back to make the computer usable, and the rest is used like a virtual memory cache, with the memory manager deciding what does and doesn't need to be written back to memory--the same way it works when you are just using computer normally.
Windows 8 even takes this further, in that it separates out two different parts of memory and hibernates them into different files. Only the part needed to have the computer running at all is in the hibernation file. The rest is written directly to the page file. In fact, the fast "startup mode" of Windows 8 is actually just resuming from hibernation using the hibernation file, but not using the page file. Hence, your friend is likely using hibernation all the time without knowing it.
The main benefit in not hibernating is just that you start out with a clean slate. (Or cleaner state in the case of Windows 8's fast start up mode). Memory does get corrupted over time. But you deal with the same issue just from leaving your computer on all the time.
The best solution is just to occasionally restart your computer. If you have Windows updates turned on (and you should) you probably do that anyways.