Apply greencard for a parent with Pre-existing condition

[eyeedea]

I am a US citizen and my parents are currently visiting US on a visitor (B2) visa. I am planning on applying for their greencard however my father has moderate -severe Alzheimer's. In an interview he will not be able to answer the questions.
Will there be an issue in his greencard application? Please advise what we need to do so that he has better chance of getting the application approved. 

Is there anyone out there who has been through similar scenario? Please help.