Spirit WILL cancel your return leg. Here is their policy:
2.4 Cancellation of Reservations
2.4.1. All reservations and seat assignments are subject to cancellation without notice if:
a. The customer does not have a printed boarding pass in-hand at least 45 minutes prior to the (original) scheduled departure time for all domestic flights, and 60 minutes prior to the (original) scheduled departure time for all international flights including U.S.V.I. flights.
b. The customer fails to make themself available for boarding at the gate at least 15 minutes prior to (original) scheduled departure time for domestic flights; or 30 minutes prior to the (original) scheduled departure time for international flights even if the customer has already checked in for the flight at a location designated for check-in. In the event of a delay, customers are recommended to remain in the gate area for updates and possible early departures. Spirit shall not be liable to any customer who misses a flight, which departed earlier than the estimated departure time posted for the delay.
c. The customer fails to travel on any flight segment of a booked itinerary and fails to modify/cancel their reservation prior to the time limit outlined in section 3.3. In such instances, all subsequent flight segments on the itinerary will be cancelled.
d. Such action is necessary to comply with any governmental regulation or direction, or to comply with any governmental request for emergency transportation in connection with the national defense. 6
e. The customer has been informed that he/she is not permitted to purchase transportation from Spirit.
Sorry, you're out of luck. With most other airlines I'd say you might be able to beg, but Spirit has notoriously rigid policies.