A coverage path with remote a remote coverage destination would be easiest. Adding a coverage time of day schedule would automate the process of switching between home and office, but I would only do that if the user spent the same days of the week at home.
Other than that, a class of service that included call forwarding would have to be assigned to her phone, and someone would have to go to the phone, dial the CF access code, and then a destination number.
Carpe dialem! (Seize the line!)