If she is going to express she needs to do it often - at least 8 times in 24 hours inc at least once at night. If she doesn't manage this then of course she will produce some, but it won't be enough long term to sustain bf and her milk supply will dwindle.
If she has been a habitual drug user, then breastmilk feeding may be the kindest thing for the baby, as withdrawal symptoms will be lessened.
I am not sure if the same applies to a heavy alcohol user.
Good luck with this.