Con men make their living by getting "strangers" to trust them!
You'd be amazed at how many people still subscribe to the:
"I'll trust anyone until they prove me wrong" philosophy.
Ever so often you hear about someone being "in love" and sending money to someone they met online but never laid eyes on in person.
Having said the above it's not about the "strangers" or "friends". It's about the individual and (their) "trust criteria" selection process.
Each of us (chooses) our own friends, lovers, and spouse.
We get to choose who we're going to trust and whom we won't trust.
If someone is constantly being betrayed or double crossed by friends or strangers it's probably time they re-examined their trust criteria process. Figure out what made them feel they could trust that person.
One has to allow people to (earn their trust) over time by demonstrating honesty and integrity. Be careful who you say "yes" to.
"In order for someone to ride your back (you) have to bend over."
- Martin Luther King