I say lost. Too bad.
Here is the fine print on the back of a TITO I forgot to cash ($.89):
"This ticket is a bearer instrument. [Casino] is not responsible for lost or stolen tickets. The player is responsible for checking this ticket for accuracy, including the date and amonts shown, and must immediately notify and attendant of any error. Tickets are void if illegible, altered, counterfeit, incomplete, produced in error, or fail any validation testing. [Casino] shall not be responsible for the issuance, validation, or payment of lost, stolen, counterfeit or multiple cash vouchers. [Casino] shall be discharged from any and all liability upon payment of any claim arising from the redemption of the first cash voucher presented with a valid bar code. [Casino] reserves the right to withhold validation and payment pending a determination whether the cash voucher has been stolen or lost. Redeemable only at [casino]."
So, maybe if they'd caught the lady, with the ticket in her possession, called their attorneys, taken it to court 2 years later, and the judge determined your friend's $100 was indeed "stolen", the casino is then "not responsible for....payment of...stolen ticket." Your friend could then hire an attorney and file a civil suit against the lady for the $100. Your friend might get a judgement in his favor, and then good luck getting the lady to fork over the $100. Of course, meanwhile a ton of time and money is spent to get the $100 back. And not likely the casino would even get involved in the first place, as was proven when they told your friend "sorry."