Quoted By:
She-Ra Ep 5, "Battle of Bright Moon": Bright Moon is the first kingdom on Etheria freed from the Horde.
Ep 6, "Duel at Devlan": Frosta just spent the night at the main Rebellion camp and there's never any indication that she's a new monarch whose first act was to rebel against the Horde.
Conclusion: Frosta was a villain who switched sides between Eps 5 & 6.