Genesis 1:11
Then God said, Let the land produce vegetation: seed-bearing plants and trees on the land that bear fruit with seed in it, according to their various kinds. And it was so. (12) The land produced vegetation: plants bearing seed according to their kinds and trees bearing fruit with seed in it according to their kinds. And God saw that it was good.
1:29
Then God said, I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food."
Your friends are entitled to their own opinion. However it appears to me they are another victim of D.A.R.E.
I say this because they clearly stand on the "Anti" side of the argument. They see MJ not a plant with medicinal properties, but as a bad street drug associated with crime.
If they looked at this more, (AHEM) scientifically, they would notice that the headache medicine they use, the coffee they drink, the aloe in their kleenex; all these are drugs and all are derived from plants.
Not to mention unless your friends are SUPER religious, Beer, wine, tobacco; All drugs and all from plants.