It's her place, you can't really be forceful on this. But you could try asking
the owner if there is anything she thinks needs to be done behind the bar that
perhaps you could take care of for her. For instance, if she goes to count the
register mid-shift or do the inventory of the bottles, you could volunteer to
do it yourself and give her a report.
You can't force your boss to do anything, but you can try and change their
perspective by offering to help.