Understanding their situation
Seeing someone we care about becoming increasingly distressed, depressed or acting out of character can be a very painful experience. It can leave us feeling helpless. We might be tempted to jump in and try and sort out the situation without thinking our actions through. Or we might simply not know what to do. These feelings can be further amplified when the person doesn't reach out for help, or engage with treatment.
You don't have to be able to solve a person's problem or even to understand it fully, but listening to what they say, and being patient, will let them know you care.