Eklavya story is quite controversial, the things go like.
And hearing reports of his skill, kings andprinces, desirous of learning the science of arms, flocked to Drona by thousands. Amongst those thatcame there, O monarch, was a prince named Ekalavya, who was the son of Hiranyadhanus, king of the Nishadas . Drona, however, cognisant of all rules of morality, accepted not the prince as his pupil in archery, seeing that he was a Nishada who might (in time) excel all his high-born pupils.
But, the Nishada prince, touching Drona's feet with bent head, wended his way into the forest, and there he made a clay-image of Drona, and began to worship it respectfully, as if it was his real preceptor, and practised weapons before it with the most rigid regularity. In consequence of his exceptional reverence for his preceptor and his devotion to his purpose, all the three processes of fixing arrows on the bowstring, aiming, and letting off became very easy for him.
Arjuna context:
Arjuna, in particular, thinking all the while, O king, Ekalavya, saw Drona in private and relying upon his preceptor's affection for him, said, 'Thou hadst lovingly told me, clasping me, to thy bosom, that no pupil of thine should be equal to me. Why then is there a pupil of thine, the mighty son of the Nishada king, superior to me?"
On hearing these words, Drona reflected for a moment, and resolving upon the course of action he should follow, took Arjuna with him and went unto the Nishada prince.
Drona answered, 'O Ekalavya, if thou art really intent on making me a gift, I should like then to have the thumb of thy right hand.' "Vaisampayana continued, 'Hearing these cruel words of Drona, who had asked of him his thumb as tuition-fee, Ekalavya, ever devoted to truth and desirous also of keeping his promise, with a cheerful
face and an unafflicted heart cut off without ado his thumb, and gave it unto Drona. After this, when the Nishada prince began once more to shoot with the help of his remaining fingers, he found, O king, that he had lost his former lightness of hand. And at this Arjuna became happy, the fever (of jealousy) having left him.
So he did not actually eavesdrop but I think I can buy the idea of eaves dropping. That could be a logical reasoning considering Dwarparyuga
Drona may have done it for Arjuna but would I blame Arjuna=== NO!
Because he did not ask drona to do that whatever was talk was it was between Drona and eklavya or eklavya and Arjuna
But that depends on perspective I guess.
.