I have used my Macbook (remember the retired white one?) for about 5 years. It has not given me as much grief as my PCs in the past, and even with its scars and spills and gross level, I can't bear to replace it until it actually dies forever. It is not as fast as it was, obviously, but it is still working and that's more than I can say about my previous PCs after 3 years (especially HP which lags like nobody's business; my previous company had a contract with HP and all our laptops were replaced and ugh, hated it).
I don't like wasting time on disk fragmentation and all that weird stuff that improves the computer's work. I just want it NOW, and the Mac is okay for that. It may only be 13" but I finished my degree on it and did a million things on it that the size no longer mattered. And the screen resolution still looks better than some of the new PCs, or maybe that's me being biased.
That said, the hard disk crashed twice in 2 years (before introduction of Time Machine), so make sure she has an extended warranty and schedules updates with the Time Machine properly! Again, USE TIME MACHINE!
My brother has a new MBP whose solid state hard disk crashed in 5 months, FYI. It was caused by bad memory sticks the reseller upgraded for free at point of sale, and an entire semester of art essays and art work lost because he did not know how to use Time Machine.