A half marathon course is really long. The vast majority of the spectators gather near the start and finish, with very few in the middle of the course. If you're going to pull a fast one, you'll do it somewhere in the middle.

Runners don't know each other - and they aren't paying attention to each other, for the most part. It's probably not difficult to duck off to the side and walk away without arousing too many suspicions. Most folks would just assume you have a cramp or something, maybe need to stop for water or the bathroom. When she gets back on the course, she'll likely be all by herself in 2nd place. Probably not a soul around.