specbill

Something to consider....since this is a new experience for both parties and clearly you want her to enjoy the stoker experience, I would offer that you reconsider your specific cadence goals for now and increase the attention you are already giving to her comfort. Specifically ignor your speed/cadence desires and totally focus on paying close attention to reading just how much she can do physically and do whatever it takes to insure she continues smiling right from the first pedal strokes. If anything, under-perform compared to what you might like to do, particularly on day one. If you can get her off the bike happy and comfortable that first day and have her sincerely looking forward to getting on the tandem the next day...you will be a real hero.
Consider that if someone hasn't ridden in a long while, facing four days of riding a tanden just may be be a bit intimidating, not to mention a bit demanding physically.... and that is aside from any medical challenges someone might also be dealing with.
Constantly communicate with her about what you are doing or what you are about to do so she gets no surprises and starts to feel confident in what is coming at her...frequently asking if she wants more or less speed...cadence up/down...or whatever. Seeking that feedback from her sure can't hurt until you are dailed into what she can handle and enjoy. Error on the side of going too easy...having her tell you to 'step it up a bit' or her telling you to 'butt out because she's fine. so quit asking' ...would be a very good thing.
Wishing you and her the best of time.

Bill J