Just for the heck of it, I checked Treasure Island's websites for room rates -- cheapest I could find for a room tonight (short notice) is $139 per night. For many of the more prestigious hotels, unless they have a lot of rooms available, they will charge you a lot more if you walk in off the street.
I can't say whether the Strip hotels would accept cash -- some "off-Strip" motels I'm sure do. If you reserve via internet or telephone, you have to provide a credit card unless this was business and your company had a corporate account with the hotel.
If she did stay at Treasure Island and she had only $150 or so on her, and she paid cash for her room, that wouldn't leave her a lot of money left over. No...more I think about this, the more I believe this was pre-planned.
Something just doesn't add up with this story, even if she did have $250.