Was there any drug available at that time that:
a) would fit Reet better than possible, but seemingly unlikely methadone,
b) was/is called very similar to it?
If her friend wasn't herself a drug user or a pharmacist, it would be imo, incredibly easy to either:
- hear Reet mentioning that she's taking "metolazone",
- see that drug name at her place
and misread or misremember it as "methadone", just cause she heard of methadone before, but never heard of metolazone before.
It would be a different story if similar hints would come from other sources or if we could reasonably expect from Reet's friend having enough experience to be sure that it was 100% sure a methadone.
I'm no expert, but it seems highely unlikely.
So young girl manages to get addicted to some hardcore drugs, while living in area where they aren't that big of a thing yet (possible - obviously), keeps it a secret from everyone/most people around her (still possible) then decides or is forced to try rehab with methadone, and side effects of that aren't manifesting itself in noticeable way (doable, possible, but bit far fetched)...
Or, cause of the market, that methadone is the drug, only available for her and she's taking it to get high (possible)...
But then, she moves to Los Angeles, where lots of drugs is available, where hippies are everywhere, where she is, being young and pretty, very likely able to get access to it, invitations to parties... and somehow resists? Stays sober? Or takes drugs for some time, but keeps herself in perfect health?
Possible, but what are the odds?
IMO much, much lower than chances that her friend ended up with false information: either misremembered, misheard or learned that from source that she assumed could be trusted but that wasn't the case.