The first rule in medical diagnosis is: When you hear hoofbeats, look for horses, not zebras. So here we have a patient who is exhausted post-transplant, and everyone is theorizing that the cause must be lack of sunlight or depression or lack of exercise? Those are zebras, since they don't have any special connection with the patient, and instead we want to look for horses, which are things that do have to do directly with the patient's medical history.
I have read of some transplant recipients who have spent as much as a year 'recovering' from the transplant, though I was able to return to full-time employment two weeks after my transplant and everyone's experience is different.
Since the overwhelmingly most common cause of post-transplant tiredness is low hemoglobin, I would suspect that first. Even though in theory the new kidney should normalize the hemoglobin levels, in fact many do not, usually because of the trauma the graft undergoes in the transplant process. The patient says her bloodwork is normal, but she should make sure that that is not just the breezy remark of some careless physician who prefers to regard a hemoglobin of 110 as normal enough, even though it is not. If you find that your hemoglobin in anything less that 120 (12 on the other scale used), insist on getting a prescription for long-term acting EPO, such as Aranesp, and you might feel a world of difference.