She went to school and is a qualified nutritionist now.
They can't fire you for drugs if you agree to go to rehab.
Addiction is a medical condition.
Firing someone for a medical condition is stupidly litigious ground to tread upon.
To be fired for drugs she would have had to have been caught, gone to rehab, gone back to work and then relapsed and gotten caught again.
Either she's the least stealth drug user in the world and they caught her on the first day of filming, or maybe the third day when she shared some bad acid with Genevieve which is why she really ran off set and never came back?
I googled Jennifer Lien + Drugs, and this thread was at the top of the results page.