I looked up your mum's symptoms on-line and could only come up with sites that discussed liver problems, some of which (the hep's) are very common in drug users.
At risk of offending: Does your mother desire extra attention from time to time, in the form of sympathy? Do these attacks coincide with events in your life, or close relative's, that might distract you (or them) from her momentarily?
I only ask because I could find no disease in my, admittedly, quick search, that mentioned vomiting colorful bile. It is possible, however, that if she has liver disfunction due to the drug abuse, or indeed has hep., she might drink a little alcohol or take some drug that has this affect on her from time to time.
I will keep looking for answers for your mum and come back with any information I find that may be helpful. Good luck to you in your search. It is very sweet of you to want to help her.
Try misdiagnosis.com for vomiting black bile...it will give a fairly comprehensive list of possible causes and links to more information that relate.
Hope it helps!