I had a student once that didn't fly in ten years. I set up a lesson plan that was about 10 hours of flying and about 20 hours of ground. I treated them like a mini private pilot student. The first couple lessons were just about getting him back in the saddle by doing basic air work and flying the pattern again. The following lessons got more complex with cross country flying, night flying, hood flying, emergency procedures, etc. By the time they were done with training, he felt safe and confident.
For ground school, since they have most likely forgotten a lot and are very hazy in rules and regs, you must do a lot of reviewing. Hit all the points that are required for your private pilot written and make sure you assign homework for them to do reading on their own.
Good luck.