I was writing about this the other day (
link), but I think if there's time travel in play and this is "Future G'raha" that we're dealing with - whether we go to his time or he comes back to ours - then it's not necessary to have completed the Crystal Tower raids.
He is from a point in time where those events have already taken place, whether you have personally experienced them yet or not.
If players haven't completed the questline yet, I think it could be easily sorted out with some variable dialogue at the start - he'll recognise and greet you, because he's been through that story with you, even if you-at-the-current-time have never met him. So your character would react in one of three ways: (for those who haven't started yet) no idea why this unfamiliar person knows you; (for those partway through) confusion because you
do know him but not like this; and (for those who completed it) full recognition and wanting to know how he got out of the tower.
That would lead into a customised explanation of what's going on, possibly with some "you mustn't speak a word of this to my past self next time you see him!" warnings for those who haven't finished.