Generally the restaurant would be in their legal right to require the last person to leave to cover the bill. Unless you've pre-established each person is paying for themselves, or who is paying.

Largely it will come down to the remaining persons actions. Are they willing to wait for the police, are they willing to give the police any information they have on the ghosting individual, are they helping or hindering the restaurant from getting the thief? The restaurant is in its legal right to treat you as an accomplice.

If you were frauded by someone. you can take them to court. If you have text based communications it will be easier, but even just asking the restaurant for video(s) and having your receipt could win a case. Most restaurants would gladly help you in this regard.

Having worked in a few restaurants I only had it happen once, and we only charged the remaining guy for what he ate, and banned the other guy. (pulled picture from camera.) It was a $85 loss, but we weren't likely to get the guy by calling the cops, court fees, and such it was just easier to take the loss. Though we did make a report with police and have him half-assed trespassed. (The statue of limitations is 3 years for petty theft in my state, and if he comes back in that time we have the incident report number so he can be charged for the theft.)