The AR difference between Light and Heavy gauntlets and boots isn't as extreme as the chest piece. If you have to have her in heavy army (WHY?) then just upgrade the heavy armor and she'll wear it as soon as it's AR is more than her leather armor (to HER, based on her skills).
Nah im not saying she needs to be in heavy armour but followers auto equip items that are the most powerful, i found it odd when she eqiupped the gauntlets and boots bur not the chest piece, but ive just smithed a full leather armour set to fine and gave it her and she equipped none of it even though its light armour... Tempramental [censored], she worked fine in my other play througs.