Incorrect translation?

The flash card says "Yo soy su amigo."  The translation is "I am her friend."
Shouldn't it be "Soy su amiga"?   Why is "Yo" added and why was the masculine version "amigo" used?

Hola GaryF26,
 
Yo is used for emphasis. If the speaker is a male it's amigo and if  a female , it's amiga . I as a male  am someones amigo. Referring to my friends:  Él es mi amigo. Ella es mi amiga
 A woman and a man : Ellos son mis amigos. Hope this helps.

Saludos,

Ricardo