I know what you mean. I'm feeling really stressed at the moment, mainly due to the fact that I raced through VCE like it was a 100m sprint and now I'm practically in holiday mode. I have no motivation at all to do homework. All I can think about is going out to parties and nightclubs with my friends who recently turned 18.
I want to get a part-time job as well, but my parents won't let me until I've finished my exams (which I probably shouldn't complain about because it would only add to my stress), so I've been using up a lot of money from my bank account. In the past 4-5 months, I've spent about $500 from my bank account on a whole lot of useless stuff that I didn't need: food, alcohol, online subscriptions, computer games etc. I was lucky that I didn't buy Diablo 3 because it was hard enough for me to quit my addiction to Diablo 2...
If mental health is a mundane problem, then my worries are probably the most pathetic excuse that anyone could think of. I have no reason not to succeed except for a lack of willpower. I've never really been one to do homework on a regular basis. I had really bad study habits from years 7 to 11, and no matter how much I try to change, they are staying with me through year 12 as well. Being invited out to places every second week doesn't help. Last year, I barely saw my friends and spent most weekends at home. This year, I've been going out heaps of weekends. Why couldn't it be the other way around?
I'll try not to go off-topic too much, though: lozmatron, you are a very nice, caring and considerate person, and I'm sure that your friend would appreciate your company very much. Whilst I may not be the best person to offer advice, I will try to give you some: don't forget about the big picture. In everything you do, you are working towards something great. The small things do matter, though they are just building blocks in a castle. Your friend's worries can be fixed, and I hope that she can recover timely and understand all of this.
When it comes to life, mental health is more important than VCE. Getting a bad ATAR score isn't going to kill you, though clinical depression might. It is important to remain calm and occasionally do something that is not school-related in order to give your mind a rest. Do not stay up too late either: having 5-6 hours sleep every week night isn't particularly healthy for your mind or body. When things seem hard, take a break from what you are doing to contemplate matters, then come back to them later when you are feeling well. Taking a break is not the same as quitting. Do not quit: only allow yourself a sufficient amount of time to recover. This applies to everyone, not just you and your friend.
Good luck and take care.