Answer: MUST not steal the drug. Maybe...
Step-by-step explanation:
Post-conventional moral reasoning is the highest state of moral reasoning according to Lawrence Kohlberg that occurs when a person has interiorized their own personal set of ethics and morals in their own behavior.
If Heinz consider that stealing is inmoral no matter the situation, and he has postconventional moral reasoning, then no matter which opportunity to steal the hypothetical drug, he won't steal it.
In this particular case, because we don't know how Heinz concieve the act of stealing, we cannot really stablish Heinz conclution.
However, most of the time the ethics and morals of those with post-conventional reasoning agree with social practices and laws.