We only saw him for two hours at Christmas time. On my Mother’s Day my brother got me drunk and I signed over my entire inheritance to him under influence and threat.
What can I do about this to make him liable for everything including what was in the safe deposit box?
Who is the executor of the estate? Is there a will. That's all that matters.
See an Elder Law Attorney or a Trust and Estate Attorney with the will, or prepare to open probate for your Mother's estate.
Don't drink with enemies.
You mentioned threats. What kind of threat? Even so, this situation sounds very sketchy to me. How was your friend being forced to get drunk? None of this posting makes any sense to me.
Nevertheless. I wish your friend well and I am sorry for his loss.
If it is you that got drunk and signed over your inheritance well...that was a pretty expensive drink.
IF it is the friend you can contact whoever is POA and they can look into this. But if you have no legal stance there is nothing you can do other than report it as Abuse of a disabled person.
If your friend has been diagnoised with a Dementia he cannot sign a contract. He needs a lawyer.