She should figure out how long she would like the short sleeve to be. Then look at the pattern and see if you can extimate the number of sts at that point in the pattern, cast on that number of sts and work the remaining increases up the sleeve. Knowing your row and stitch gauge will help with this estimate.
If the pattern calls for the sleeve to be knit from the top down (or if you can modify it to knit from the top down), it's all even easier. Pick up sts for the sleeve, knit down to the desired length and then bind off.