I have an obese client who I train twice per week for 30 min. sessions. Twice she has expressed to me that after our workouts she is so damn tired the rest of the day. We are only doing 30 mins and it consists of beginner exercises such as BW squat, res. band presses, & TRX rows. I review her food log weekly and her diet is very good, as well as her post-workout meal. I think a good part of it has to do with sleep issues.
For someone obese these exercises can indeed be too much. A squat alone puts major stress on their spines and knees. What seems like a beginner exercise to us is high intensity to the obese. She is telling you that it is too much for her. How about giving her fewer reps and more active rest intervals such as easy biking on a recumbant bike (if she is very obese even walking may be too much). Sleep issues can certainly be part of it, but you want her to feel better after being with you, so I would suggest easing off. If she cannot afford to train with you extra days as Harris suggested see if you can get her to do at least 10 minutes of activity on her own on off days to help her build her strength and endurance.