From personal experience, I would say a PC would be better (if there's enough room). Laptop keyboards can be a bit small and, as buteman says, touchpads are not as easily used by the more senior. Whichever you decide to go for, though, the cheap end would be more than adequate these days. I would suggest a check of the local libraries, colleges - some of them do a basic intro for those of more senior status who have not used them before - takes the fear out of it.
Try the local "freecycle" website in your area. As fruitbat said earlier, there are lots of older PCs being given away free, and she wont need the latest up-to-the-minute model for basic working. One request on the site should evoke a response.