Most airlines will provide a full refund, upon request, if flight times change substantially (such as more than four hours). The passenger can then book flights on another airline.
Unfortunately, you might find that Spirit Air does not offer the same level of customer service in such cases. At least that was our experience.
About five years ago, my wife had a similar problem with Spirit Air. They cancelled her flights after September 11, and they tried to put her on flights at completely different times that conflicted with her work commitments. Their website said they would offer full refunds for flights that were cancelled after September 11. The first agent cheerfully said that we would get a refund in ten days but no refund showed up. So we called back. The story changed. A series of Spirit Air customer service reps and supervisors refused to provide a refund. They said that because Spirit still operated a flight between our home airport and the destination airport, her flight wasn't really cancelled by their definition. Some were quite rude, including one who said, "You OBVIOUSLY didn't read the conditions of carriage," when I politely pointed out the what their website said.
They did allow her to travel on different dates because of the September 11 situation, but the whole experiennce of dealing with Spirit Air was quite unpleasant.
I have very low regard for Spirit Air after this experience.