Answer:
In order for the daughter to have any recourse against her father spending her inheritance, she must file a motion in court to have her father adjudicated incompetent. Adjudicated Incompetence requires the daughter to prove that the father does not have the ability or fitness to manage his affairs due to a mental condition determined in a court proceeding.
Explanation:
The standard for Incompetence is extremely high. There are not enough facts given to provide a clear answer as to whether the daughter would be successful in such a proceeding. First, as we get older, people have a degree of forgetfulness. This does not necessarily equate to mental incompetence. Second, there are many facts presented that indicate the father is mentally capable. Such factors include participation in community events on a daily basis and driving to places he desires. The fact that he may have physical health issues (this is only implied in the question due to the later description of memory loss) is of no importance since Incompetence focusing on the mental condition of the father.
There is no law requiring the father to refrain from dating a woman half his age nor is there a law requiring the father to live in a manner that maximizes the daughter's inheritance.