We train primarily on the tandem but my stoker was working out with a heart rate monitor before riding her single or tandem. That said I think that she has a heart rate zone only attained when we are chasing a single.
Everyone is different so what will help your stoker train is difficult for anyone here to know. Some are competitive, some need a long term goal, some like long slow distance of touring or brevet pace. My only suggestion is to try every type of ride and if she likes one go down that path. The goal is to promote a desire to improve.