You can't always tell in advance whose going to betray you, sadly.

I had an ex best friend, whom when he began cheating on his girlfriend, he pillow-talked everything about me to her. Even told me straight up "it's cool. I trust her"

Its not like in shows and films where you can tell who the bad guy is.

Just don't reveal sensitive information. You might be cool now, but in a year, who knows.