A few thoughts occur to me.
Did OP's father have
mental competence at the time he opened the account ?
In other words, did he have
contractual capacity, at the time of opening the account, to enter in to a contract ?
If not, all that followed the opening of the account might be null and void.
The contract thus formed might be regarded as void ab initio i.e. as if it never existed.
There is an issue of credibility. There is a principle from the law of equity that says that he who seeks equity must do so with clean hands. I do not question the integrity of the OP's father, especially where mental health issues arise. The conceptual difficulty is that the conduct of the account holder leaves him wide open
objectively to adverse inferences being drawn and all of the consequences that may flow.
BTW the presence of mental health issues at the relevant time does not automatically mean that his conduct is excused.
What has to be established, by evidence, is that the depth and nature of the mental state caused impairment of mental capacity at that time.
I am sympathetic to EBS. They probably feel, with justification, that whilst there may have been procedural weaknesses they still acted in good faith and might be compromised whatever decision they make. I am amazed at the notion that there is nothing that they can do. If OP's father sought relief in the courts and orders were made against them EBS would have a perfect solution as would the account holder
.
There have been suggestions that OP's father should instruct a solicitor. I endorse that very strongly because many of these matters have the capacity to become an utter morass if they are not resolved definitively.
The Financial Services & Pensions Ombudsman Scheme might be one way to go to resolve this. If you submit the issue to them for adjudication I understand that their rulings are legally binding on the parties. Alternatively, the court route might be considered. Neither option is perfect as each has potential disadvantages. The FSOP option does not expose you to costs
unless the other side appeals and thus puts you straight in to the High Court.
FSOP link
https://www.fspo.ie/
I hope that OP can get this sorted sooner than later and that there will be one major worry lifted from his father.