A few tips from a therapist:

Never tell a person having a panic attack to “relax.”

Never tell a depressed person to “snap out of it” or that they can just change their thoughts.

Never tell someone struggling with an addiction to just use more will power.

If you don’t know what to say you can ask if there is anything you can do to help. Sometimes it helps to quietly stand bye. Sometimes you may need to leave people alone.

If you really can’t identity with the depth of someone’s pain or despair it’s OK to appreciate how fortunate you are; but that might not be the right time to say it out loud.

If someone is really suffering and you are worried about them, keep in touch with them. If they need more help, help them find it.