my niece's Frontier flight got cancelled after the pilot brought the plane back to the gate due to mechanical problems.

The next available Frontier flight was 3 days later. The agent gave he a full refund on her ticket and a $100 Frontier voucher.

Unfortunately, the next available flight was on AA with a connection, the next day. The price was $370 more than her original Frontier flight. She had no choice but to pay it.

Is she entitled to reclaim the $370 from Frontier? Also, does Frontier not rebook passengers on other airlines under any circumstances?

Thanks!