You’ll also want to take into consideration how often they’re currently running, the longest distance they’ve run in the past and how long ago that was, and the longest distance they’ve raced. You’ll also want to know about any previous injuries or structural abnormalities (flat feet, high arches, bow legged, etc). These things will change the frequency they may be able to run, as well as what type of flexibility and strength work you’ll want them to complete. Also, check with your client as to the amount of time they have to devote to training.
A half marathon plan should not only include a running plan (miles, intensity and frequency), but also a flexibility, strength and eventually fueling/hydration plan. Often when people train for their first half, they focus all their effort on running. Getting the distance in is one part, preparing the body to travel the distance is another…and travel the distance injury free!
Active.com has a lot of free training plans for all distances as well as tips on strengthening and flexibility to prevent injuries and prepare the body for the run.
Good luck! You’ll both learn a lot in the process! I know ACE and AFAA have CEUs on training for runners.