I don't think there is any way to get the backup job to initiate as soon as teh drive is there, as apart from anything else Backup Exec won't keep polling to see if it has been connected, so won't know if it's there or not.
Your best best is to set it up as a removeable backup to disk drive, point your backup job at it, and then determine a time when you'll be sure the office manager will have connected it to the server (allowing time for her to be late or what ever obviously), so it can then run automatically each day, and be completed by the time she leaves and disconnects it again.
If you don't want to worry about checking it all the time, you probably also want to ensure you check the option in Priority and Availability to automatically cancel the the job after a period of time to ensure it has been cancelled and re-scheduled for the next day in case for some reason she doesn't connect it in time one day.
