So, I don’t know why people are getting confused and thinking the hero cursed the girl?
It would appear that when casting a high-level curse, it requires the sacrifice of an item of some sort. The amulet with the shape-curse seemed to have disappeared while the girl was transformed and only reverted back after the curse was destroyed.
I imagine it’s the same thing with the ring. My guess is that the hero used the ring as the catalyst for the hex on the MC, to either then kick him out of the party to take all the glory or to then use it as an excuse to keep the MC back from getting into further danger at his age.