It depends on your local authority as to whether or not the case will come to court. Some (mine!!) rarely prosecute but other are a bit tougher.
In my LA if there are genuine reasons why she is not attending, bullying, mental health issues etc.. AND the parents have been co-operative, attended meetings, done everything they can to get the child to school then the judge will usually throw the case out. Therefore the LA don't prosecute as it is a waste of everyone's time and money.
But if she is refusing to attend with no reason and parents are ineffective then there is a much higher chance of prosecution. Its usually a fine, very rare for a custodial sentence. And don't forget its a court case where both sides are heard, the LA and the parent, its the judge who decides the outcome.
From the info you have posted it seems that your friends DD is a child with very complex needs who should be receiving lots of support from the school's pastoral team and Learning Support Dept (for her ADHD).
I would suggest to start with a multi agency meeting with everyone, school, EWO, social services, SENCO to put a personalised package together for her although this might have been done already?
There are often off site vocational providers eg post 16 colleges/trainer providers than run courses she might find easier to access but usually school would have to pay for this.
She needs to have an adult in school who she trusts and can build a relationship with, a learning mentor/behaviour support worker, head of year?
She is obviously very unhappy, perhaps a referral to Child and Adolescent Mental Health Service (CAMHS) would be appropriate, it is usually quicker for GP to do this.
And it is illegal to exclude a child for non attendance.
HTH and tell your friend the best thing he can do is be co-operative, honest and make notes of everything school have offered, they should be doing everything they can to get her in before court is raised as an option.