There was an exchange on twitter about this issue a while ago:
mominem @mominem Sep 3
@FrontierCare If a passenger misses the outgoing flight on a round trip do you cancel return flight?
Frontier Airlines @FrontierCare Sep 3
@mominem Yes, please check out cancellation at:
http://spr.ly/6019ByZbP . ^BG
mominem @mominem Sep 3
@FrontierCare If I cancel the outgoing flight is the return flight still valid?
Frontier Airlines @FrontierCare Sep 3
@mominem If you cancel the outbound prior to take off, you can still fly the return. ^BG
mominem @mominem Sep 3
@FrontierCare So if I cancel and Economy ticket I will have a credit for 90 days. If I use the credit I will then pay a $99 Fee. Correct?
Frontier Airlines @FrontierCare Sep 3
@mominem Yeah, there would be a change fee and applicable fare difference. ^BG
Here is
the applicable part of the T & C
VOLUNTARY TICKET CANCELLATION
<snip>
Economy tickets will retain its value for 90 days, and the value of your ticket may be applied toward the purchase of a future ticket subject to a change fee of $99.
Never tried it but is seems you can cancel a ticket and not pay the change fee but if you reuse the credit that's when you pay the $99 change fee. Of course if the ticket is less that $99 the credit will simply expire.