A good Tripod/head is an excellent idea and having had a Manfrotto 055CB for years can fully endorse that model
Good quality circular polariser filter is a good extra too, find out what lens she uses for landscape type shots and buy one in that size, Hoya Pro are excellent quality (not from Ebay, possible snide copies)
Not everyones thing, but find a wireless remote very useful, bought a Hahnel Giga Pro, loads of functions, good range and nicely made into the bargain
Fast high capacity memory card is another idea along with spare camera batteries, always useful
One last idea how about a days photography course in her favoured subject, days tuition from a pro can make a world of difference