I have to agree with the 70-200/2.8 IS II note (above). If she already owns this lens, then given her main subject matter, I can't think of any Canon primes that would provide a different/better outcome except maybe the 100mm macro, and even that doesn't fit her subject matter unless she wants to shoot more closeups. The 50, 85, and 135 are all awesome lenses, particularly the 85/1.2, but I think they are more useful for other kinds of work than high mountain landscapes.
If she does not own the 70-200 IS version II, well, that's my most used and useful lens, and a substantial upgrade over the original 70-200/2.8 and the I.S. version, both of which I own. The only hesitation I would have is if she owns the 70-200/4 IS, which is also a wonderful lens, and which I find easier to carry and use when out for longer periods of time. If that is the case, then the 135/2 might provide a significant difference in the look and feel of the images when shot wide-open.
What does she have now for longer lenses?