I would simply have helped her as you did and maybe made sure she got a taxi but other than that there is nothing else you can do.

She may not have been on drugs it is just as possible she was suffering from a mental illness, or she could have been homeless and ashamed to admit to it.

There are also many legal (medical) drugs that can permanently stain your teeth not to mention heavy smoking or coffee drinking - both of which you find in many psychiatric patients.

The fact is - you see a person in distress you have to act on the information you have.

If she did take the money for drugs then you are not responsible. You saw a person in need and you did your best to help them.

That should be the end of the story really. No reason to feel bad or ruminate over it.