Is a person considered sinful or as having fallen into doubt (shubha) if he stands up for a disrobed woman to sit in his place on public transportation with the intention of lowering his gaze from her, knowing that his intention is sound? And will he be rewarded for that? And what if there is a veiled woman and a disrobed one, and he stands up to seat one of them, which of them should he prioritize over the other for the sake of lowering his gaze and for reward?

If you stood up for that girl with the intention of alleviating evil and reducing corruption, we hope you will be rewarded for that.

However, you should be careful not to expose yourself to situations of suspicion and ill-conjecture.

And if we assume there is a veiled woman who is upright and a woman who adorns herself immodestly, then what should happen is that the outwardly upright veiled woman should be seated according to Islamic law, for several reasons:

Seating the immodestly adorned woman is a cause for suspicion about you. Seating the immodestly adorned woman might increase her temptation and encourage her to persist in falsehood. Not seating the immodestly adorned woman contains a form of deterrent for her.

People of righteousness and religion are more deserving of honor and veneration, so alleviating the burden of standing for the outwardly upright veiled woman is more appropriate.

And you should forbid lowering the gaze as much as you can.

So, if you seated the veiled woman and forbade those who gaze at the immodestly adorned woman, you would have combined both benefits.