Hey @rd - I'll try to take off my moderator hat and answer your question
I can definitely understand your dilemma. On the one hand, you want to uphold your business model, encourage client responsibility, and avoid setting a bad precedent. On the other hand, you probably just want to be helpful and provide a service to a customer who's in a bad spot. Tough decision.
I think the best solution would be to let the client know exactly what you've said (that you would have definitely had the backup if they had had a maintenance plan with you), that you generally don't have backups that long, but you're willing to check to see if you have it on the off chance that you do. If you do have a backup, that means you've been storing their website for them for quite some time. I'd focus on that as a service, come up with a fair price, and ask them to pay it. I'd make sure to emphasize exactly how lucky they are that you have the site at all and encourage them to reconsider a maintenance plan too.
Anyway, that's my two cents.
Puts mod hat back on.