philip, I for one cannot imagine being able to tumble in full plate - you might well roll to the floor easily enough albeit with a bit of a clatter, but wouldn't it be something of a struggle to get back to your feet?
It wouldn't make sense if heavy armour didn't penalise your acrobatic type thieving skills. That's why heavy armour has a high dexterity penalty, try picking up a real chain mail shirt sometime and see how agile you reckon you'd be wearing it
So no you can't tumble in heavy armour. Stick Dorna in some nice leather or studded leather armour and she'll be just fine, unless you've told her to gain levels in cleric instead of rogue.