my 15" albook is quite silent still, although the hard drive WILL get noisier in time, when it's used. mine is on almost 24/7 and it is already noticeably louder than half a year ago. it will need a hard drive replacement within a year. but still, quiet enough, and the cooling fan is not a problem.
the tibook, however, is noisy by default. at least my old 667mhz dvi model was, because the cooling fan had to operate almost all the time. the reason for this was the stock toshiba 4200rpm hard drive, because after i changed it to a hitachi 7200rpm drive (which itself was quieter, too), the thing ran MUCH cooler and the cooling fan almost seemed to disappear. after the drive change i had to stress the cpu at +80% for some time to get the fan operating!
so for the original poster: change the hard drive of your mom's tibook to a new one. it will help a lot in regards of noise levels and hard drive performance. the stock drive is too slow and needs to work more than necessary, so it overheats. i recommend buying such a drive that has 8mb cache buffer and at least 5400rpm rotational speed. it is really worth it, and the drive is relatively easy to change by yourself if you can handle a screwdriver.