One way to handle these damsels in distress is to tell them the truth.
"I don't help girls I am not fucking."
Effective Truth is better than effective altruism. After all, had this been a man, he would never even have entertained the thought of approaching you, a stranger, for 20 francs. The answer to that one is automatic: "Ha ha! no, get the fuck outta here".
So you know the situation is implicitly a sexual one already. You're helping a damsel, due to sexual reasons. Let us then make it explicitly sexual:
"I don't help girls I am not fucking."
The truth feels great. To have all that sexual "heroic man saves girl in subway" anxiety go away in an instant, plus to see the look of disbelief, changing to respect, in the druggie's eyes. Her "easy mark" turned out to be a lion in disguise. Be prepared to be propositioned for a 20 franc blowjob right there and now, though, and to decline it with perfect gentlemanly charm and no mention of the yellow teeth.
You're such a piece of shit.
Thank you very much.
No, I don't have that kind of bitterness in my heart towards my female friends. Besides which, the friendzone is a misogynistic concept driven largely by male entitlement.
I don't need to be bitter about not having sex like you apparently do either, I have had plenty of partners all over the gender spectrum.