I have no problem watching something for someone. It is human nature, we all like to help when we can. There are limits, in this case 15 minutes is fair. If I agreed to watch the laptop and it was time for me to leave I would go to the establishment manager and tell them what happened and then leave. It then becomes the obligation of the establishement, regardless of what the employee would like you to think.
Lets Keep it Simple
I have no problem watching something for someone. It is human nature, we all like to help when we can. There are limits, in this case 15 minutes is fair. If I agreed to watch the laptop and it was time for me to leave I would go to the establishment manager and tell them what happened and then leave. It then becomes the obligation of the establishement, regardless of what the employee would like you to think.