First of all, please be aware that there isn't much you will be able to do to help this person if she isn't committed to losing the weight. You can't be with her all the time, so maybe she's eating/drinking more than you know.
I once tried to help a very obese (5', nearly 400 lbs) person to lose weight; I gave her ideas for food, exercise, etc and she assured me, she was doing everything I suggested, plus exercise and I believed her............. until I walked into her office one day and she was sitting there eating a candy bar, with a can of coke........ that wasn't one of my recommendations. She said she only wanted a "treat", but golly, I caught her the next day with a different kind of candy bar and another soda. This happened numerous times, plus I'd watch her eat at meal times; she'd eat nearly 3 times as much as I'd eat and I ate too much......
If your friend's parents aren't supportive enough to provide proper food and exercise options or encouragement, this person will have a very hard time.
All of that said, Ducky gave you some very good advice. What I would add to that is to have your friend get her thyroid tested to make sure she isn't hypothyroid, which causes weight gain/inability to lose. She needs to be tested for a minimum of TSH, Free T3 and Free T4........ by all means she should get tested for diabetes, because excessive thirst is a major symptom. Being overweight, is a major symptom of type II diabetes.
Unless she's drinking an obscene amount of water, I wouldn't worry about the amount. 1-2 liters is recommended but I once talked to a soldier who said that on active days he would easily drink a gallon of water without a second thought. That being said, his active days were really labor intensive and if she's constantly thirsty I strongly recommend seeing a dr asap because that's a big sign that COULD (not always does) point to diabetes. Given her weight, it's quite possible for her to have diabetes and she needs to make sure it's being watched.
Second thing you need to tell your friend- loosing 30 lbs in one month is not healthy, safe or realistic. Loosing weight so fast can make someone extremely sick so no matter what they do on tv (and remember they have dr's on all those weight loss shows making sure no one is going to die). It's just not really safe to do this at home. Good, safe weight loss is about 2 lbs a week, no more. Usually your first couple weeks you loose a couple pounds more, but 2 lbs is a safe average to maintain.
Third- real weight loss is hard. It takes strong control to keep up a healthy lifestyle and avoid going overboard indefinitely. (Which is the goal, you have to make a PERMANENT lifestyle change to avoid regaining the weight.) You need to have the education as to what is good food and good amounts; the will power not to eat more than is necessary (not eating till your full, but satiated); and the determination to stay firm and focused when you hit a plateau (which is bound to happen). So there is nothing that will make loosing her extra hundred lbs any easier and she has to want it enough to go after it, not just wish for it.
When I was at 230 my knees, back, and feet got sore and tired very easily. It's a lot of pressure to carry around so much extra weight. Because of this walking may be too hard on her body, but if she feels up to it that's something the two of you can do together. Have her walk with you 30 minutes a day for 4-5 times a week (although starting out at 3 might help her at first). Let her set the pace and keep in mind that there is a big difference between being sore because you worked out and being in pain because you did something wrong. So if she complains of PAIN, then walking might be too much for her. Ideal exercise for her would be swimming. It puts much less pressure on your body and makes it easier to move all your weight around while getting your heart rate up. However, pools are expensive at it sounds like you guys are teens -so unless she/you can convince her parents to pay for the YMCA you may be out of luck. Also because you sound young, perhaps you can take PE together. It's much easier to be "pushed" by a friend, then it is to be made to run faster by the teacher/coach or feel everyone is waiting on the fatty. It would likely help out moral to have a friendly face in such a setting.
Now it sounds like she's not getting much help from her parents, perhaps she should see a dr. Have him lay out in black and white how much it will effect her health to her parents. Her parents may be overweight themselves and feel like that's ok and normal. It's not. I'm a round 195, and I have butt loads (sorry keep seeing your name and giggling) of health problems because of my weight. I'm 24 and have half the health problems people start dealing with at 40 because I spent my entire life overweight and obese. Because of her weight she could also convince the dr to send her to a dietician (and have medical insurance pay for it). So that is a theoretical option she might want to try out. None of this will be easy for her to hear but, if she isn't having the family support at home, maybe this will open her parent's eyes to how much harm this weight will do if she keeps it and get them to help out.
ON top of all this she has to make sure she's eating right. It's sounds like the first thing she has to make sure she's doing is avoid fatty, fried, packaged foods. These include obvious fast foods as well as things like generic white bread; Potato chips as well as frozen lean cuisines. Most of this stuff has tons of refined and processed sugars/ syrups, starches and just more junk than anything remotely healthy. It's hard when you're not buying the food, but she shouldn't be eating this kind of foods. But for good idea of what not to eat- if it comes in a wrapper, box, or on a Styrofoam plate it's not really healthy. Second she needs to eat ALL 3 meals plus a couple snacks. If you starve your body it will hold on to weight and make it impossible to loose anything (not to mention you will suffer from malnutrition). Yogurt, fresh fruits, nuts, low fat dairy, boiled eggs- these are all very good healthy snacks (when in moderation). For dinner time have her try to fill up half her plate with veggies, a quarter with lean meat/protein (broiled or baked chicken, fish, shell fish, beans, eggs, etc.), and a quarter with a whole grain (brown rice, whole wheat bread [not the cracked stuff at walmart mind you-because that's still got glucose syrup and sometimes high fructose corn syrup], barely, etc.). This isn't always easy, but its a good visual at about how much of what she should be eating.