Whatever I could do to help the person with the flames on their back as they would be the immeadiate priorirty but at least I wouldn't stand around doing NOTHING. Taking them to the ground from the bottom beneath the flames and dragging them out of the carriage might have helped while protecting their face and hair from going up or literally tearing the clothing off them if possible while using whatever means available to smother any flames left. Literally whatever was humanely possible and as quickly as possible. If there were already people actually getting the job done on her and I was aware of that guy being the perp (which in the confusion may not have been possible) I would have asked another commuter to assist in restraining him until the police were there to make an arrest or try to knock the fucker out with whatever I had from a piece of furniture in the subway or a beer bottle or something out of a thrash can. I don't know exactly but I 100% would have done whatever I could. And what would YOU have done exactly?
See? There's another one of your problems. Freedom of religion is possibly the strongest of the freedoms, and is in the 1st Amendment.
The guy and the woman were simply of a different religion than most of us. And he was carrying out religious punishment on the woman... punishment that she was aware of in her religion, and deserved for doing wrong.
All you would be doing is breaking the 1st Amendment by disallowing them their freedom of religion. You should be punished under the law for trying to steal someone's freedom. You even admit that you were trying to do this.