"He defeats his friend."

Translation: Ele derrota seu amigo.

[gerry11111]

couldn't "vencer" be used as well here?

[Paulenrique]

Vencer we often use to "win"

[Kate911634]

Not amigo dele?

[MarcelJay]

Can't it be amiga?

[Luis_Domingos]

Yes, but you also need to change the possessive: "sua amiga". If that wasn't accepted, please report the whole sentence as being correct the next time you run into it again.

[Ray.Kuryla]

Why was "o seu amigo" wrong?

[Paulenrique]

It's also right.

[SpencerPri4]

Why was is not amigo?

[Luis_Domingos]

"Amigo" is a correct answer. Maybe you made an error in the sentence around it :)