I am so sorry that you have entered the nightmare world familiar to so many of us with ruined lives.
There is much on this forum to help you, look at the impulse control disorder thread and google Dopamine agonists and OCDs.
The legal question...no you can't sue. Look at the question and answer session on this website.
No legal firm will take on the drug firms on in this country although there have been many successful cases in other countries.
The might of their superior funding and amorality enables them to escape any responsibility.
On a practical level, your Dad must come off the drugs.
His neurologist must be forced to face the havoc the drugs have caused and supervise withdrawal.This is not easy but essential.
Your parents GP needs to be fully involved and can arrange counselling.
Citizens Advice can help untangle debt problems.
The Parkinsons helpline can offer support and advice.
I share your anger.
The facts about these drug side effects were known for years but kept from us.
Publicity and acknowledgement was too little too late including from PDUK.
No support for legal action was offered before the 3 year rule ruled it out.
Many of us limp on , our money gone, our relationships shattered, but we live some sort of life managing to find a little pleasure again. I wish you and your family all the luck in the world...you are going to need it.
Read as much as you can and find out as much as possible about the effects and the possible ways forward and then approach the professionals.
Keep posting and let us know how you get on.
We are thinking of you and always here.