Dronacarya did not hesitate to impart all his military secrets (Chapter 1 Text 3)
Chapter 1: Observing the Armies on the Battlefield of Kuruksetra Friends,
acarya mahatim camum
tava sisyena dhimata
pasya--behold; etam--this; pandu-putranam--of the sons of Pandu; acarya--O teacher; mahatim--great; camum--military force; vyudham--
arranged; drupada-putrena--by the son of Drupada; tava--your; sisyena--disciple; dhi-mata--very intelligent.
O my teacher, behold the great army of the sons of Pandu, so expertly arranged by your intelligent disciple, the son of Drupada.
Duryodhana, a great diplomat, wanted to point out the defects of Dronacarya, the great brahmana commander in chief. Dronacarya had some
political quarrel with King Drupada, the father of Draupadi, who was Arjuna's wife. As a result of this quarrel, Drupada performed a great
sacrifice, by which he received the benediction of having a son who would be able to kill Dronacarya. Dronacarya knew this perfectly well,
and yet, as a liberal brahmana, he did not hesitate to impart all his military secrets when the son of Drupada, Dhrstadyumna, was entrusted
to him for military education. Now, on the Battlefield of Kuruksetra, Dhrstadyumna took the side of the Pandavas, and it was he who arranged
for their military phalanx, after having learned the art from Dronacarya.
Duryodhana pointed out this mistake of Dronacarya's so that he might be alert and uncompromising in the fighting. By this he wanted to point out also that he should not be similarly lenient in battle against the Pandavas, who were also Dronacarya's affectionate students. Arjuna, especially, was his most affectionate and brilliant student. Duryodhana also warned that such leniency in the fight would lead to defeat.
Source: As It Is by HDG. A.C. BhaktiVedanta Swami Srila Prabhupada.