Yes, the ring makes this ability free. It was intended that way.
Note that her ring costs 1 more mana than other similar Mage rings. That little extra cost was just for this reason. It will take her 4 rounds to make a profit on this ring (not counting the quick spell action required to cast the ring!).
Keep in mind, she only gets the -1 mana benefit once per round. So, if she uses it with Force Pull, then she does not get the benefit to use with other force spells she casts. Basically, as soon as she puts on that Force ring, she should be prepared to cast at least one force spell per round, or else she is effectively wasting one mana.
Design note: These Mage rings were originally built-in Mage abilities. They are interesting because they help indirectly steer a mage into a certain play style and type of spell casting. Later, we decided to not force that upon the player, but instead make these into rings. However they are priced very enticingly!
