It's hard to see a loved one suffering, especially if you feel helpless. Sometimes you feel like you cannot make a difference, or that your help doesn't matter. But it does. All the help you provide to someone who is suffering from an ailment, especially those suffering from a mental disability, makes a difference. People value feeling loved and appreciated.
So how can you help? There's many ways that you can aid someone suffering from a mental disability. Often, someone appreciates just talking to a trusted friend or family member about their problems. But this isn't everyone's approach. Some people dislike talking about their mental disability, and that's fine. You should never force them into doing something that they don't want to do, especially when it comes to opening up. Doing so may worsen their condition. The overall idea to take away is that you should follow their lead when it comes to what kind of help they need.
But some conditions are different. If you feel that a person suffering from a mental disability is a liability to themselves or others, you have to get help. Even if they tell you to never tell anyone about their condition, if you feel that it is dangerous you need to alert someone who can take matters into their own hands. Along those same lines, if somebody tells you about their condition in confidence and you don't feel like it is serious enough to bring to another person, keep their trust and don't tell anyone.
Let the person talk and listen to them without judgement. Reassure them that treatment and help is available. Tell them that what they are experiencing is treatable and that [dark] feelings are temporary.