I believe in 3.5 this did not work. I can't find any ruling text in pathfinder saying that it does or doesn't.
On Magic on the school Transmutation on the subschool Polymorph says, "In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph [subschool] spell."
I'd want to verify this with your GM, but unless a spell specifically denies a particular tactic, I'd assume the player found a clever use for it and allow it.
This will work sometimes
Enlarge person can only work on a humanoid creature, see spell text. If the party member polymorphs into a humanoid creature (which is allowed), then, enlarge person will enlarge her size. Otherwise (if she polymorphs into a non humanoid creature) it will not enlarge her.