Agreed with the above. If you don't have Lancer leveled high enough for Invigorate, that's a skill you'll absolutely have to have cross classed to Bard (especially at end game) so I would make that the priority.

Otherwise if you already have LNC leveled for Invigorte, then if your friend is a THM then I would consider leveling a tank or healer... you'll be able to get fast queues together that way rather than both queueing as a DPS, and as they're fighting stuff in open world like with hunting log and fates you can either take over aggro as a tank or keep the two of you alive as a healer. If they're leveling gladiator however, then I would suggest leveling a healer. I would suggest Arcanist except for the fact that you can only be a DPS class until you unlock the Scholar job at 30, which also requires Conjurer up to level 15 to do, so if going with a healer I'd just pick Conjurer at this point.