Your friend needs to read this very carefully especially section 6.
http://www.taylorwessing.com/uploads/tx_siruplawyermanagement/Enforcing_US_judgments_in_UK.pdf
A British court may very well refuse to enforce a judgement made in the USA against a person with mental health issues ( violent suicidal) as they consider it to be 'against natural justice' In any case the friend will need to start another court case in the UK after getting a judgement in the USA in order for it to be enforced here.
I have to say that unless the son has a lot of money to pay any judgement made this is going to be a very expensive waste of time , there is no way to reclaim costs either in USA or UK in a case like this so your friend needs to have several tens of thousands of £s that they can afford to lose.
Sorry to be so negative, but a lawyer especially a US one, is not going to tell you that you are almost certainly throwing away money.
Please ! get some sensible advice from a solicitor with international experience before spending any more cash on this.