ADHD stands for Attention Deficit Hyperactivity Disorder. It basically means Lack of concentration, boundless energy. Most people with severe ADHD do end up in trouble with the police, in hospital due to self harm etc, I am not giving a bad name to sufferers of ADHD, but they are associated with bad behaviour.
From what you have told me, it is looking increasingly more likely that your friend has ADHD. If you could tell me anything about her attention span, and what she is like in class? Does she often start day dreaming and looking out of the window, half way through the class? This is important, as, if it is known that she has ADHD, you must notify your school A.S.A.P because students with ADHD are often allocated rightful extra time during exams, to make up for the attention difficulties. If she does have ADHD but the school are not notified, they cannot allocate her extra time and this could seriously affect her grades, as I see her GCSEs will be coming up very shortly.
What can you do? You can do alot of things. I suggest you talk to her, and say, "Look, I think you do have ADHD. It's not the end of the world, it is treatable! If we get you to a doctor who can give you professional help, you will be feeling alot better. I think it is also important for you to stress to her the importance of GCSEs, and that going to a doctor to get a perscription for a common ADHD medicine, such as Ritalin or Concerta, will help her in her GCSEs.