No rule for this one
No ending or word group predicts the article of Freund. Learn it as a pair and say it out loud: der Freund.
der Freund
It's der Freund. Freund is masculine, plural die Freunde.
Mein Freund wohnt in Berlin.
My friend lives in Berlin.
No ending or word group predicts the article of Freund. Learn it as a pair and say it out loud: der Freund.