If she has a digital camera, 80GB may fill up fast especially if that camera can record videos, such as Casio EX-S series in MPEG4 format so that it does not fill up too much space on the card (half an hour on 1GB card at its highest quality, it looks OK on TV.) Of course, the flip side is you end up recording quite a bit of movie clips on trips, birthday parties and what not, so you end up filling up your hard disk in the end. Still, 80GB is not that small, either.
60 is not that much of an age barrier to get into iPhoto slideshows and clips onto DVDs. Besides she will need some kind of backup mechanism. Never trust a hard disk with your memories. Once you include SuperDrive, Mac mini is not a very good deal unless you already have a nice monitor.