Is the doctor we met facing significant karmic liability by promoting cannabis?

This is quite true. As in all things, karma will out and karma will rule, because everything has consequences. Whether they are good or bad is in the eye of the beholder, but we can tell you that the many lives that will be distorted and undermined by too frequent a use of this substance will grow in numbers to catastrophic proportions when there are no legal checks and balances. This will be another learning curve akin to the cocaine epidemics of the late 1800s and early 1900s. There will need to be a re-learning of the consequences of the habit-forming substances being insidious in their undermining. And so, many will be harmed and will have lifelong, negative consequences. This will trace back to him in terms of those who are exposed and encouraged to abuse this substance through his television outreach. This is most unfortunate, but he is blind to this. Your natural impulse is to warn him, but again this is an intrusion on his freedom and will be seen so from his perspective. You can certainly state your views and make it personal with regard to your desire to not incur such debt and let him draw his own conclusion and inferences about his own fate. That way, you are not pushing something on him and labeling him, and introducing a new threat directly to attempt to frighten and manipulate. If he makes the connection, so be it, but you are entitled to answer his offer and tell him reasons why. This you can do with a suitable light touch, but get the point across. And this, as well, is serving the light by spreading the divine perspective and that is a contribution in its own right. Whether it falls on deaf ears or not will not be your responsibility. All you can do as a divine messenger is to bring the information forth and then let the chips fall where they may, so to speak.