Andi,
There has been so much great advice already that I feel I have little to add.
However, Kammy is Kammy, and David is David. A marriage, relationship, engagement, partnership, whatever we may like to term it, does not one person make.
Kammy and David are each individuals with individual weaknesses and strengths.
Kammy is showing a definitive leaning towards an addictive personality and David is showing his total dislike of this personality type wrapped up in glossy paper as the woman he loves.
David has a choice to "not" get any further involved with Kammy as Kammy also has the choice to be involved with smoking "pot".
That is freedom of choice. Kammy has her choices and David has his.
I totally agree a councilor would help, but I also question whether Kammy really wants a councilor to help her get off of the "pot".
If David really dislikes Kammy's drug dependancy; tells her so and she still continues, he should leave. Otherwise he must ask himself if he is co-dependant on Kammy's behavior patterns to the degree that he would sacrifice his own belief systems.
Just my 10 cents worth.
Ana