I used to be fast for 20 miles, then fall apart. It's a lack of aerobic base, and the only way through is to develop that aerobic base. As said above, the way to do that is some longer rides (even if not by much) that are slower than she's been doing. And, yes, eating before and having a gel or two during the ride will help immensely. It'll take some time, but she'll get there - I had to go through it myself this time last year.