Your friend isn’t subjecting herself to ruling authorities, as Paul advised in
Romans 13:1-7. It not just talks about to whom we owe taxes, but following the laws of those who govern:
(Rom 13:1-3 Douay-Rheims Version) 1 Let every soul be subject to higher powers: for there is no power but from God: and those that are, are ordained of God. 2 Therefore he that resisteth the power, resisteth the ordinance of God. And they that resist, purchase to themselves damnation.
If your friend feels that the drug laws are unjust, yu can find a little about that in the CCC
here, under Participation in Social Life:
1902 Authority does not derive its moral legitimacy from itself. It must not behave in a despotic manner, but must act for the common good as a “moral force based on freedom and a sense of responsibility”
You can explain that the government has already weighed the pros and cons of medicinal marijuana, and ruled against it.
1908 Second, the common good requires the social well-being and development of the group itself. Development is the epitome of all social duties. Certainly, it is the proper function of authority to arbitrate, in the name of the common good, between various particular interests; but **it should make accessible to each what is needed to lead a truly human life: **food, clothing,
health, work, education and culture, suitable information, the right to establish a family, and so on. (Emphasis mine.)
By ruling against marijuana use - medical or otherwise - the government is acting in the common good to a) evaluate the medical effects of marijuana, and to b) restrict access to that which is deemed harmful.