I'd recommend doing the MSQs and supplementing that with daily roulettes (and maybe a few short stints in PotD if she falls behind on the required levels).
PotD is great for getting fast XP, it's decidedly suboptimal for getting more than a general idea of what you're doing with your class.

She'll need to do the MSQ eventually so she might as well do them as she levels and use other things to shake up the flow when the story feels like it's starting to drag.
Leveling in PotD does get old after a while, trudging through that until 60 and then having to do the enirety of the MSQs when they're mostly "low level errands" sounds like it's not the best of experiences to me. We already have people complaining about having to do the 7th Astral Era Quests before getting to HW.