The Global Pass would let him travel on any date of his choice. However seat reservations could be an issue.
This plan seems rather complicated I'm afraid, at least in Spain. Pretty much all trains require seat reservations there (incl. regional trains) and they could sell out in advance leaving him stranded.
Service is very irregular across the country with a lot of railways seeing 2 trains a day.
If he's fine with booking a few days in advance it might be possible. Or he follows the Atlantic coast on little trains (which don't require reservations as far as I know).
Or he could maybe look at travelling in France and then Italy... the latter has very frequent trains everywhere.
Don't forget to book the Eurostar well in advance too.
General link :
https://www.seat61.com/interrail-and-eurail-reservations.htm