...my questions are these: what do you consider to be trustworthy traits in people? What clues or cues would you consider to be red flags as indicators of people not trustworthy? Once trust is broken can it be fixed? How are boundaries established? What about in professional settings? Can a person trust someone who is not a friend? What about therapists? Can one trust a therapist? What boundaries can be established once trust is broken? I believe trust to be a very personal and deep bond between me and someone else ...what do you think trust is?
One simple answer to all your questions is: The situation decides the person's personality.
You might have been good friends in good times but when bad times occur, there can be issues.
One more thing, Why do you expect someone to be trustworthy? Are you not strong enough to stand tall alone?
Make friends, be fun and loving but never expect anything for anybody in return.