tasyātmano ’rdhaṁ patny āste
nānvagād vīrasūḥ kṛpī
The wife of Droṇācārya, Kṛpī, is the sister of Kṛpācārya. A devoted wife, who is according to revealed scripture the better half of her husband, is justified in embracing voluntary death along with her husband if she is without issue. But in the case of the wife of Droṇācārya, she did not undergo such a trial because she had her son, the representative of her husband. A widow is a widow only in name if there is a son of her husband existing. So in either case Aśvatthāmā was the representative of Droṇācārya, and therefore killing Aśvatthāmā would be like killing Droṇācārya. That was the argument of Draupadī against the killing of Aśvatthāmā.