Not an expert, but the "I can stop whenever I want to...just not now" thing is a pretty classic addict line. Addicts, especially young ones, rarely consider themselves to be addicts. Her coke habit may not seem like a problem now, but these things have a nasty habit of spiraling. If I was in your position, I would advise her to be honest with her doctor about her cocaine use. At the very least her doctor can explain what the impacts would be on her post-OP stomach.