[twelve40]

Just plan out the time and reflect it in the grants and vesting schedule. Like, you get low x% grant for the first year because part-time, then after that first year, you become full time with another grant (with vesting, of course) that will top you up to 50%, or we part ways. Doesn't sound difficult to implement? It also sets the borders and timeline for commitment so that the person doesn't just linger around undecided.

As to whether or not go all the way to 50% in total... kind of a different question... Obviously better for you to keep as much as you can in the pool and not give away, not even out of greed but to keep the options in the pool to afford more star hires in the future, for example. But it might create bad blood if the person really comes through and starts contributing.