After a domain expired, it will pass through a number of states, before it gets deleted. The last state before the deletion is the Pending Delete State. If the domain reaches this, the previous owner can not renew the domain and it will be deleted after exactly 5 days! (for COM/NET Domains) It also gets removed from the zone file.
There is no priority system for Domains in Pending Delete. Not for the previous owner and not for the previous Registrar. Everyone can register it when it gets finally deleted and everyone has the same chances. It only depens on the system you use to automatically register those domains. Some Backorder companies have more Registrars and with that can do more Registration attempts per second/minute. These have the best chances to actually get the domain.
Conclusion: The Domain will definitely become available and the previous owner can't do anything anymore to keep the Domain (except backorder of course). Who gets the Domain depends on the quality and quantity of resources a Backorder company or an individual puts into it.