If you are the friend of a depressed individual, especially one who has opened up about having depression, there a number of things you can do that will not be helpful, and you should do your best to avoid them.
First, while you should encourage your friend to seek professional help, you should be aware of the way in which you do so. Not everyone needs an intervention, and you should be wary of reporting a friend to college or a professional.
However, there are a number of seemingly less extreme things you might do that can be harmful. Do not be critical or judgmental in listening to your friend, even if you may not agree with what they are saying. Though you will not have all the answers, do not shut them out or refuse to listen to them. Avoid telling your friend to cheer up or snap out of it; most depressed individuals know they are depressed, and telling them to get over it will not help, as they do not usually have control over their feelings or downturns. Avoid sounding ignorant, as saying ignoring things will reinforce negative thinking. Don't try to talk your friend out of their feelings either, even if they are irrational. Depending on the situation, it may be best to keep suggestions and solutions for another time in favor of simply listening and letting your friend talk and vent. If you invite your friend out and they continually refuse, keep trying but do not push or make them feel guilty for not attending. Do not distance yourself or cut off communication. Asking a friend not to tell you about their depression, especially if you have already offered to be a listener for them, is hurtful. Do not see your friend as troublesome or a burden if possible; it helps if you are able to separate the idea of your friend and their illness.
Most importantly; don't stop being the friend.