>Second, I think the "money well spent" part is that you helped her in some small amount, when it was obvious she was suffering, and you didn't expect anything in return.

Let's be clear here: the "out of gas" story was almost certainly a lie and OP knew it. He helped her buy a small amount of drugs.

I have a sibling struggling with drug addiction for the past 6+ years, constantly "borrowing" money from others and never quite hitting a serious enough "rock bottom" to decide to get help/rehab. My brother "borrows" $10+k/year from my dad and it's poisoning their relationship and draining my dad's finances. (my dad recently took out a 401k loan because he's so cashflow-negative and that money's gone already) My dad enables him every day while dreaming about moving to another state and cutting off all contact.

$40 for OP to cut ties with someone like that is indeed money well spent, IMO.