I think leveling all 6 jobs to 15 would feel a bit time consuming to someone still new. Having leveled all the jobs out of the dunes or higher except rdm(mnk isn't in my sig but I have it to 24 on a galka mule), I would say monk would be the easiest. Easy to solo, straight forward to xp with at lower levels, can view others. If she does go for mnk, I recommand buying her a hand-to-hand piece with a high DMG rating, and don't worry too much about delay at lower levels. It helps keep the numbers higher than 0-5 per punch which is something I see bad starting monks commit a lot. And also I recommand buying her meat jerky stacks, which are cheap. They're great for melee damage jobs early on.
War is impacted a lot by not having a subjob in my opinion. One would miss out on a bit of vit/defense/hp. And such dunes parties are known to be really ugly sometimes, everybit really helps. The lack of a subjob won't impact whm,blm,and mnk as much in my opinion at lower levels so I'd choose amoug those 3.
Thf is just a pain in general. But if she wants to be a thief, I'd recommand coming back to it after getting a SJ to ease the pain of leveling it. It sucks a ton until 15. And I also recommand using a sword for thief until 30ish(33 mainly) and then switching to dagger.
Some people that tend to like all or most of the jobs, such as me, are going to have a really difficult time picking a main before even experiencing the game. However, after leveling a job to 18ish and being in a few parties, a person will experience for his or herself how the game generally works on a basic level. Once a person sees how the game works, they'll likely have a better idea of what job or jobs he or she wants to do.
I'm just thinking about trying to explain FFXI to a RL friend that has never played the game before. Someone in my LS said his friends call it" FFXI, the forest stimulater". My brothers friends call it "magic bean farmer". Now, we know FFXI is pretty good game or we wouldn't be playing it. But explaining FFXI in all its detail to someone new can be tough.
So in a nutshell, I think getting blm,whm,mnk to 18 would be for a smoother ride, especially blm and mnk as they're not very high pressure jobs starting out. But as far as a main job goes, she's probably best finding out for herself from watching how parties work 10-40. That way you and her won't make a bad choice or pressure her, and she can discover the game and community for herself.
I think it's awsome though you're getting her involved. If you're there making/joining parties with her, that'll probably help her out a lot since she can ask you basic questions. There is a lot to take in from this game at first. My LS has 4 married/soon to be married couples in it, and they seem happy sharing the game together, so good luck.