Well first things first, make sure that they know what Data Center and Server you play on. When they create their character, being on the same server will help a ton with playing together. From there, a conversation would be best. Do they want to do the story side-by-side with you? Do they want to be as independent as possible? Are you going to play at about the same rate, or will one of you spend much more time in game? And as an MMO vet are they going to come in with an open mind, or with a ton of pre-conceptions of how things should be?

So from there you can examine your options. If they want you as close as possible then either an alt or a NG+ run would be ideal. In either of these cases youll be able to see and interact with all the quest NPCs again, see the cutscenes again, and replay any solo duties again. If they're fine on their own and likely only need help for the big moments then let them do their thing and just join for the sync'd dungeon and trial runs. You can make either of these easier by playing a class that compliments whatever they pick. Being close to level 60 means you could easily unlock one of the later expansion jobs that start at 30/50/60 if you need to swap roles(and thus save some time leveling up).

Beyond that, just be ready to show off side things like the Gold Saucer or the Wolves Den. Theres plenty of fun to be found, but not everyone likes the same things so variety can be key.