Questions & Answers about Mój przyjaciel pomoże ci.
Because mój has to agree with przyjaciel.
- przyjaciel is masculine singular
- so the possessive pronoun must also be masculine singular nominative: mój
Compare:
- mój przyjaciel = my male friend
- moja przyjaciółka = my female friend
- moje dziecko = my child
So mój does not depend on the speaker’s gender. It depends on the noun it describes.
Specifically, przyjaciel is a male friend.
Polish usually marks gender in nouns like this:
- przyjaciel = male friend
- przyjaciółka = female friend
So if you are talking about a woman who is your friend, you would normally say moja przyjaciółka.
Also, przyjaciel often suggests a fairly close friend, not just an acquaintance.