You do not need to have enough 500-milers available to support your upgrade to request it, just when you check-in.

So he can put in a request for both legs and if he clears the outbound and the return, you can then ask to see if the agent will let you sponsor his outbound with your certs so he can use his for the return.

If his outbound doesn't clear, then he will have enough for the return.

And worst case, he can buy four for $200 to cover both segments (with spares).