After taking 2 levels of Abjurant Champion, she can qualify for this class. If she continue to be a mage/rogue type, this is not a bad choice indeed. Don't forget to take Practiced Spellcaster feat anyway.
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Depending on how far you want to take Abjurant Champion, if you take the entire 5 level progression you will get the martial arcanist ability which allows you to use your BAB as your Caster Level. So based on those two things, very difficult to dispel with a Shield spell that gives a +9 shield bonus. And being able to swiftly cast Abj. 2nd level and below. Or do you round up? Which would be 3rd level.