The use of plants as a metaphor for showing caring between two friends is a good one, and your initial designs seem to be on the right path.  It seems to me that the your "competition" is a cell phone, since it's easy to call a friend and directly tell him/her to cheer up (assuming that you have a way of knowing your friend is feeling depressed).  So, as you refine your interface, continually ask yourself what you're providing that a phone call wouldn't provide (or at least how you provide a different type of experience). 

It's not clear to me that people would be willing to wear an EEG cap all the time, so I wonder what other ways there are to detect that someone is feeling depressed.  I'd imagine that there's actually some literature on the topic, so it would be worth digging around and seeing what you can find out.