Thanks for your reply.
The law is that a joint account automatically passes to the survivor, upon the first party's death.
I don't of course know the full history of the reasoning behind the setting up of the account, but if for example all the money was your Dad's, the fact he has put it into a joint account implies there was an element of a gift to his lady friend, as your Dad would have been advised that if he were to die first, the monies in the account would pass to his friend.
Unfortunately if a gift is made then it cannot then be retracted, and unless you have evidence to the contrary (ie the account had been set up fraudulaently or evidence can be produced to show that if your Dad did place all the money into the joint account placing the money into a joint account was not intended to be a gift to his friend), you ar enot going to be able to pursue this issue.
Therefore, without very strong evidence, it is assumed that the monies should and will pass to his friend, and to be honest, you will find it very difficult to prove otherwise.
I am sorry this is not the answer you were looking for, but it sets out the legal position.