So did you have an official contract, equity deal, etc? I doubt there's much you can do about these decision not to take you on after all, you probably need to grieve for the job and move on, but you should certainly hold them to their legal obligations, especially if they had to give you notice (which they should be paying you for).
Even with an official contract, unless he was explicitly promised certain things, he can still be let go.
American companies generally don't have any notice requirements. The fact that he's overseas may create a liability for the company in that country, but they probably don't care about that.
(edit: They must absolutely pay him for hours worked.)
Assuming his options had vesting (which 'tptacek will remind us to always have, so the YC company probably knows about that advice), he doesn't have any equity.
I've seen this problem from a few sides, so OP has my sympathy, for whatever it's worth. I've seen people laid off a week after joining when the adults suddenly realized they were bleeding money. I very very recently saw someone approved for a job and accept it, told to wait a few days while internal funding moved around, and then suddenly the entire project was cancelled.
So did you have an official contract, equity deal, etc? I doubt there's much you can do about these decision not to take you on after all, you probably need to grieve for the job and move on, but you should certainly hold them to their legal obligations, especially if they had to give you notice (which they should be paying you for).