The symbolism is wonderful in the construction of that spell, and your good intent is more than enough to cast it with or without the rest.
The thing that really needs to be mentioned, however, is that depression is not just feeling sad sometimes. Depression can be caused by a lot of other circumstances surrounding the person, or it could be a chemical imbalance. It's wonderful of you to care and try to help, and sending positive energy is always nice, but you have to understand that depression is not something that can simply magically go away, no matter how hard you wish for it.
I have dealt with depression for the better part of ten years now, and I have found a lot of ways that help me personally to manage it, but it's not something that just goes away becauseI want it to.
The best thing you can do is let your friend know that you are there for him. Invite him out with you and your other friends, invite him to hang out with you. Let him know that you care. Show it, don't just say it. There is nothing wrong with sending positive energy and love either, but if he doesn't know that you're doing any of that or looking out for him, it's not something he can really tangibly think of.
At the end of the day though, depression is a mental disorder, not an emotion. It's wonderful that you want to help him, but he may need to speak to a professional, especially if you think he would ever try to hurt himself or someone else. I have found a lot of things that work to manage mine, but they don't work for everyone. Some people find that medication works better. Others find that therapy is really helpful.
Again, it is really great that you want to help, and you are doing the right thing by being there for him and thinking about him, but this is alsonot on you or your magical ability or the spell you cast if it doesn't magically disappear, simply because that's not how depression works.
Best of luck to you and your friend