couldn't "vencer" be used as well here?
Vencer we often use to "win"
Not amigo dele?
Can't it be amiga?
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.
Why was "o seu amigo" wrong?
It's also right.
Why was is not amigo?
"Amigo" is a correct answer. Maybe you made an error in the sentence around it :)