Theophylact of Ohrid

So when his fellowservants saw what was done, they were very sorrowful, and came and told unto their lord all that was done. The fellowservants are the angels, who are shown here to be haters of evil and lovers of good. They do not tell these things to the Lord as if He were unaware of them, but in order for you, O reader, to learn that the angels watch over us and are angered by man's inhumanity.

Commentary on Matthew ↗

Augustine of Hippo

By the fellow-servants is understood the Church, which binds one and looses another.

Catena Aurea by Aquinas ↗

Thomas Aquinas

And his fellow servants seeing. Here four things are touched upon. First, the reprobation of this sin is presented; secondly, the reproach of the sin on the part of God, at then his lord called him; thirdly, the punishment, and his lord being angry, delivered him to the torturers; fourthly, the similitude is applied, at so also shall my heavenly Father do to you, etc. He says therefore and his fellow servants seeing, etc. …

Commentary on Matthew ↗

Remigius of Rheims

Or perhaps they represent the Angels, or the preachers of the holy Church, or any of the faithful, who when they see a brother whose sins are forgiven refusing to forgive his fellow-servant, they are sorrowful over his perdition. And they came, and told their lord, what was done. They came not in body, but in spirit. To tell their Lord, is to show the woe and sorrow of the heart in their carriage.

Catena Aurea by Aquinas ↗