May I give an (admittedly completely amateur and likely off-base) psychological take on this?
Friend has, according to the OP, an ex who was seriously involved with drugs. Friend has probably heard lots of justifications for his destructive behaviour. She's heard him claim that his drug use was "completely harmless," while feeling that she was, personally, being harmed a lot.
Then, she hears the OP defend a drug, possibly using some of the same phrases her ex has used to her. Her anger, hurt and disgust at her ex are transferred to the OP, tarring her with the same brush. It's irrational, but it's the result of being hurt.