I mean that they have a fall back in the sense that they can always just defer their acceptance or re-enroll (if they dropped out) if their plans don't pan out. Colleges are generally pretty forgiving about things like that.
I'd be surprised if the fellowship terms didn't include something about irrevocably turning down the acceptance and officially leaving. (Which of course is not to say that they couldn't re-apply - I rather suspect they'll have a better shot the second time!)
