How to cope with family issues

[Samsonov]

I am on a sort of crossroad over here, in regards to my approach of dealing with my family members, whether I should take a more active approach and try to help to what i believe will be best for them, or let them be.

It predominantly has to do with my father.
My father is the epitome of stage orange, a successful businessman, a rationalist , materialist who puts on a show to hold his social status.
He is in his seventy's, and is the most stubborn person I know, reluctant to make any sort of change in his life because he has got it all 'figured out'.
In his prime, you couldn't blame him for holding this position, he has founded a successful business, was very rich and had a beautiful wife, which was responsible for the beginning of his downfall, when she divorced him, after meeting another younger man, his self-esteem went down, and trying to hide the effect it had on him, he tried to maintain his old life just the same, as if it didn't even bother him. to this day, almost a decade later, he is still living in the past.

This is where I com in, im trying to persuade him to changing things up, exposing him to new hobbies, perspectives or people, but he never even gives anything a chance...
I tried talking him into changing his diet so he might feel better and have more energy, but he swears by his own trusty habits, even though he eats unconsciously and is addicted to sugar, tried to encouraging him to trying a new hobby, but he rather sit in front of the television all day and night, and now we come to the nail in the coffin for him, his girlfriend, if you can even call her that.
she is thirty years younger then him, a lawyer, married with children, and to cut to the point doesn't give a dame about him and I suspect wants him for the money she might be getting, or even worse, is planning some scheme of getting even more out of him, in short, a parasite. 
Now here you might blame me of being neurotic or overly suspicious, but this is the opinion I came to after years of knowing this person, and trying to be very friendly with her at first, so I am not making these acquisitions lightly. 
I tried to express those suspicions of mine to him, where he repeatedly explains what a good person she is, how she helping this guy who has cancer and whatnot and that he loves her and whatnot and how it is all too complicated, that why she is not leaving her husband and thats why their relationship is secret. to me his relationship with her is more of a facade, so he can showcase his young attractive girlfriend (even though he demands to keep it a secret) so his actual loneliness and inability to recover from my mother living him won't be seen.
in summary, he represent everything I hate in the world, that is why I feel like I must change him, I believe that I am much smarter then him, but the way he disrespects what I have to say and his general mannerisms make me furious, like nothing else can.

How shall I approach this/him? let him perish? how can you talk with a person who says " I am too old to change anything" , I try very hard to maintain a relationship with him, but his ego and unconsciousness are too much to bare. Just like the old world he represents, to be honest, I cant wait to see him wither away, I pray that he will find peace then, but I also suspect that he might have some more karmic work here and now, why is he still here then? I try to teach and help him, after all he gave me so much, I want to help him more then anything, my biggest wish is to see him just enjoying his breath and the simplicity of just being. You can hear and understanf the conflict I'm in, I thank anyone who took the time to read all of this and even more to those who took the time to formulate a thoughtful comment:)