Thanks to both of you for your replies.
The fact we can use the same credit card for both SE accounts is really important.
We don't really care to find "holes" in the system and exploit them, that's why I asked if he will need a new SE account. However, I wonder what's the meaning of having different FFXIV accounts registered under a single SE account (through Mog Station Account Management)? When I first noticed it I thought that Square-Enix did that to facilitate families who want to play together: one Square-Enix account for the family, and two or three FFXIV accounts for you and the kids. Am I completely wrong? Apologies for the silly questions, but I have no idea how all this system work.
We now buying a new copy of the game and downloading on his laptop. As for when Moogle world will open so he can create his character, well, that's a completely different story.
