Did Dronacharya say or do anything to stop it?
If not, why so?
Dronacharya was silent. Here's what Bhishma says immediately after the disrobing episode in this chapter of the Sabha Parva of the Mahabharata:
I have already said, O blessed one that the course of morality is subtle. Even the illustrious wise in this world fail to understand it always. What in this world a strong man calls morality is regarded as such by others, however otherwise it may really be; but what a weak man calls morality is scarcely regarded as such even if it be the highest morality. From the importance of the issue involved, from its intricacy and subtlety, I am unable to answer with certitude the question thou hast asked. However, it is certain that as all the Kurus have become the slaves of covetousness and folly, the destruction of this our race will happen on no distant date. O blessed one, the family into which thou hast been admitted as a daughter-in-law, is such that those who are born in it, however much they might be afflicted by calamities, never deviate from the paths of virtue and morality. O Princess of Panchala, this conduct of thine also, viz. that though sunk in distress, thou still easiest thy eyes on virtue and morality, is assuredly worthy of thee. These persons, Drona and others, of mature years and conversant with morality, sit heads downwards like men that are dead, with bodies from which life hath departed. It seemeth to me, however, that Yudhishthira is an authority on this question. It behoveth him to declare whether thou art won or not won.