The "son" would suggest the best translation is "she is his friend". Otherwise wouldn't the french be "elle est sa amie"?
When "sa" is in front of a word (adjective or noun) starting with a vowel, it is changed to "son", to ease enunciation (avoid the hiatus SA_A)
Ah so it's gender neutral; it could be either.
Yes, you're right, it could be either "son ami" or "son amie"
But how does one differentiate "son ami" from "son amie" in daily speech? is there difference in the pronouncination?
No difference at all. If any ambiguity with our counterparts, you can say "son amie-EU", or "son amie femme" or "son amie fille".
Not academic, but efficient...
should "she is his girlfriend" be correct?
why couldn't son mean her in this sentence?
it could indeed, "his" or "her" still translate in "sa/son"