My friend is depressed mostly all the time now?

You sound like a very sweet person and a great friend. I think you're right not to tell the guy your friend's age. That's your friend's business and it sounds like she's got a plan to tell him already. As for her depression, let her know you're there for her, but do encourage her to talk to an adult about it (parent, youth pastor, teacher, etc). Locate the phone number of a local teen crisis hotline and give it to her. If she seems suicidal or even hints at it by saying things like "I'd be better off dead", etc. then you need to tell a trusted adult. Even if you fear it may harm your friendship, her safety is the most important thing. Broken friendships can be mended. Dead friends can't be brought back. Good luck sweetie.