It says "2 hit points per level for a full 8 hours of rest in a day."
Is that the patient's character level or the healer's character level?
The context of the Heal skill makes this clear:
If your Heal check is successful, the patient recovers hit points or ability score points (lost to ability damage) at twice the normal rate: 2 hit points per level for a full 8 hours of rest in a day, or 4 hit points per level for each full day of complete rest; 2 ability score points for a full 8 hours of rest in a day, or 4 ability score points for each full day of complete rest.
It is the subject's level, as the skill clarifies it's "twice the normal rate."
It depends on the patient's, being an improvement over what he'd normally heal if he was just resting without a nearby healer doing his "magic".