Der Mann, von dem ich gedacht hatte, er wäre mein Freund, enttäuschte mich.

English Translation

The man who I thought was my friend deceived me.

Why isn’t this “disappointed” rather than “deceived?”

he deceived me = er täuschte mich
he disappointed me = er enttäuschte mich

“Enttäuschen”: literally “Ende des Täuschens”
You think somebody is nice, but he deceives you. You find out and then you are disappointed.
So in German disappointing is the end of deceiving.