Not sure what a private investigator could do when you don't even have a lead of where it went. At this point it is her word against some unknown cab driver. Would a lawsuit against the cab company go anywhere? I'm guessing not, but IANAL.
Are there pictures or an inventory of the jewelry? I would file a police report itemizing what is missing/stolen. Watch craigslist, ebay, pawh shops etc. to see if any of the stuff shows up.