can some with dementia get married

If he is mentally incompetent (does not understand what he is doing), the marriage would not be valid. Dementia *may* result in mental incompetence, but merely having dementia does not automatically result in a finding that a person is incompetent--it depends on what their level of understanding and thinking is. Also, the marriage will be presumed to be valid if he was not determined by a court to be incompetent prior to the marriage (and a guardian appointed) or there is no after-marriage determination or finding made by a court, in response to a challenge to the marriage's validity, that he was incompetent at the time he entered into the marriage. That is, "incompetence" is based on someone's medical and cognitive or psychological status, but is a legal determination or adjudication made by a court based on evidence of the quality of the person's thought processes; someone is legally considered to be competent until and unless a court finds him otherwise. If you want to look at what would be required to find your father incompetent and block this marriage, consult with an elder law attorney.