I'm sure I remember a newspaper article from at least five years ago, the gist of it being that Whitney had a (then secret) habit a long time before she met BB. When she did meet him, he was an easy route for her to take, both because he had a well established supply of drugs and also because his bad boy image would be an easy explanation of her habit if ever it came to light.
Having said that, I believe that even if he did introduce her to drugs, she chose to take them, so she is ultimately responsible.