I don't believe that ANY drug dealer would allow so me one to run up $25k debt. They couldn't afford it, and it wouldn't be tolerated. I think the figure she was given to buy drugs has been grossly inflated. If she was in fact given money to procure drugs and take them back to Alice. In my opinion anyway.
Yes, the drug dealers do let druggies run up debts. Maybe they pick a person (someone in need of drugs for themself) on purpose sometimes to let them run up a debt, so they can control the person in debt to them..get them to do the risky work - picking up drugs, dropping them off, drug courier.
If they don't do it, most likely they will end up dead and dumped somewhere isolated.
The drugs would have cost the dealer far less than the street value of them.
Here is an example ($30 000 debt)
http://www.wavefm.com.au/news/illawarra-newsroom/90796-berkeley-hairdresser-sold-drugsto-pay-debt