Although the possessive adjective would be "son" in both cases, the noun would be different - "ami" instead of "amie". So you know the friend is a woman in this case. Of course they're both pronounced the same, so if it were spoken rather than written then "you cannot say" would be the correct answer. 

Sylvie parle à son amie. (female)

Sylvie parle à son ami. (male)

Thanks Alan, now i get it.  In a similar kwi zig question:  le professeur parle a son eleve, the eleve could be male or female. Oui?