Stay with your family or fulfill a dream? What would you do if you were terminal and only had about a year to live?

Personally, I would find things I wanted to do, that included my family.
I could not be selfish with my remaining time and leave my family...I would view every minute as precious and spend as many as I could with my family.
But my right is not everyone's, your friends right is not everyone's so he will need to do what is right for him.

I have had to deal with questions like this a lot as a minister and the truth is that there is no right answer. There are lots of wrong ones and some that are almost right but when you are dealing with end-of-life the goalposts never stop moving.
The best any of us can do is to strike a compromise.
With regard to your friend in particular -- he has a family so he is not the only one in the equation.
The best he can do is to smooth the path for them after he is gone. If he satisfies his desire (at least to a reasonable point) and also meets their need to have him close as long as possible, he will be doing well.
How that works itself out will have to be between him and his family. There's no way for any of us -- even you -- to know enough about that dynamic to propose a full solution.
The truth is that there will be a lot of healing taking place if they work it out between themselves with love, understanding, and a sacrificial nature -- each trying to give the others what they need.
I am praying for them.

I wouldn't advise him. I think the decision is between him and his family. Just my $0.02