Leveling to 60 is pretty fast, so in most cases, you’ll be 60 before you’ve finished a single expansion. Also, doing all the expansions is a huge undertaking. I think when I did it during BfA it took me about a year, and I wasn’t using Chromie Time, so I was one shotting everything.
Next, she’ll need to actually get access to Chromie Time. I’ve seen people say that creating a trial character works to unlock it for new players, so give that a try. (If she bought Dragonflight, running an evoker through it’s starting area should also get you to 60 fairly quickly. (Otherwise, she’ll have to level once to 60 without it, which means BfA or Shadowlands for at least the last 15ish levels.)
So, I’d probably pick an expansion and just do that without trying to do them all. I’d go with MoP, WoD, or Legion as they’re all pretty good leveling experiences with less of the quirkiness of earlier expansions that may be frustrating to a new player. But, if you have a personal favorite expansion, it’s fine to go with that as well. (Your passion might be contagious!)
If you do want to try to play through all of the expansions, you’ll want to lock your levels a few times. Here are some considerations on what levels to lock at:
I’d lock at 49 at least until you’ve finished MoP. That will minimize phasing issues with BfA end game content. You may be okay up to 58 with the Dragonflight changes to BfA, but 49 is safer and there’s no reason to be higher than that before you’ve finished Warlords. (All of the Warlords and lower content should be soloable at 49 with Chromie Time off if you want to do raids and such.)
Legion group content will not be soloable at 49, so at some point you’ll want to level up to 58 and lock again. (I’ve seen some reports of Chromie Time being weird at 59, so I’d lock at 58 to be safer.)
The Shadowlands starter quest can be hard to pick up if you can’t get into the right phase. That basically means that you’ll want to pick up the quest when it’s still easy to get, so when you still have access to Chromie Time where you can choose Shadowlands to force the quest to pop.
You’ll lose access to any non-BfA dungeons in the dungeon finder at 60 and beyond, as you need Chromie Time to access them. You’ll lose access to BfA dungeons in the dungeon finder at 61.
I do not think you can currently access the XP off NPC after level 59.