Non-compete clauses are very limited and hard to enforce in California (where most Google employees reside.) Assignment-of-invention might apply but if the individual has left the company (rather than building it in their free time while still employed) it would probably be very hard to prove that the original idea happened while they were still in the employ of the company, unless they documented it and shared those docs internally. (i.e. you can't pitch the idea internally, and then if rejected quit and build it yourself.)
