To the best of my understanding, current law clears both of you to use (but not distribute) in that situation.
You lawfully purchased your CDs, and then made fair use of your property by ripping into a format convenient for your needs. You may continue to make use of your lawfully purchased property even if you are no longer in physical possession of the original discs. The analagous situation would be if you’d made a mixed tape (for example) using CDs you owned and then your house burned down. You could still use the mixed tape even though you were no longer in possession of the original CDs. Or if you’d made a photocopy of a book strictly for personal use and then the original was destroyed somehow. It is fairly well established by the case law in the area that making a personal copy of a copyrighted work for personal use (as a backup copy, in a mixed-tape situation, etc) is not a violation of the exclusive rights granted by 17 U.S.C 106. Provided, of course, the copy is really only for personal use (like ripping your own CDs onto your computer for your convenience - rather than ripping them to distribute them electronically to the world ala Napster, for example).
It might be problematic for you to prove that you at one point owned the CDs should it ever come up for challenge, though. Of course, it will also be difficult for them to prove that you didn’t own the CDs (as long as you ripped from your own CDs as opposed to downloading from shared sites).
The waitress is a slightly more difficult case, in that she did not purchase the CDs personally. However, given the facts you provided, she did come into possession of the CDs lawfully. She didn’t purchase them, but neither did she download pirated versions of the copyrighted work from teh Intarweb. The Copyright Act actually provides for an owner to resell their property. (If interested, see 17 U.S.C 109.) I would imagine this situation would be viewed as a solitary resale (even though no resale actually took place). This is not because sale actually took place but because the net effect on the copyright owner is essentially identical. A transfer of copyrighted material between the original purchaser and a subsequent owner.
Provided that the waitress doesn’t proceed to distribute the tunes on the CDs far and wide, there is no difference idealogically between the lost-CDs situation and the original-purchaser-resale situation. In either case, the copyright holder has only been afforded their royalty once (at the time of original sale).
This is essentially where copyright law intersects with property law. One of the basic tenets of property law is that to have a meaningful property right, one must be able to dispose of one’s property via resale (this is referred to, for complicated linguistic reasons, as free alienability). The US legal system frowns on things that put limitations on free alienability of property. Hence, copyright protections are limited to allow free alienability of the personal property (artwork, books, CDs, etc).
The Copyright Act therefore does not prevent the purchaser of copyright protected material from selling his or her property. However, this does mean that if you resell the property (like the CDs), you have no right to make further claims congruent with ownership. For example, if your hard drive crashes out after you misplace your original CDs, you don’t get a pass on re-downloading your songs for free because you bought them once. That might be ethically acceptible, but not legal application of the concept of “fair use” in this situation.
This is a situation in which property law > copyright protection. A person in legal possession of copyrighted material is entitled to enjoyment of that property. Even though there wasn’t a sale in this case, the waitress came into possession of the CDs in a lawful fashion by following all established procedures to reunite found property with its owner. Property law treats such situations as though the waitress had legally purchased the property in an original purchaser resale, so copyright law is compelled to treat it likewise. In other words, her title to the property is good, therefore copyright law is obliged to treat her title as good and afford her all the rights and responsibilities of an owner of copyrighted material.
So, to sum up, you can use your ripped files and she can use the CDs secure in your adherence to the law. Hopefully, it wasn’t too confusing!