My boyfriend is 100% disabled, and he "takes care" of an old female friend who has medical issues. I am concerned because he pays her bills ($1000 + for her electricity), he is giving her his 2012 car, he pays her 2014 city taxes of $1000. He often sleeps in his car and eats ramen noodles. I am concerned that this woman is taking advantage of his extremely good nature, but if I go to authorities, he will know I turned her in. What can I do?

Turn her in for what? Nothing you have said about her is criminal.
I'm sorry that this is tough for you, but people have to live their own lives. He seems to care about her enough to sacrifice himself for her. I'm not sure where this leaves you - for instance, why he is sleeping in his car instead of in your place. If he is so broke taking care of her that he can't even contribute to your shared expenses with him, or the most minimal of times together, and even mooching off of you so that he can support her, then you need to break up with him.

Go to the authorities for what? Nothing he is doing is illegal.

If you can't handle this, why do you stay with him?

To quote Joypulv, "turn her in for doing what"? Maybe he is a strong believer in "Do unto others", the "Golden rule". Dates back to the Babylonians.

And maybe he believes in paying it forward.

Is the boyfriend disability mental? If so, perhaps she is taking advantage of a mentally disabled person, in such case, perhaps he needs to be taken to court for a hearing to see if he is competent or not.

But, if just physical disability, you may want to consider why he is a boyfriend, if he is doing all that for another women.