It was my impression that Lactulose is for the confusion that is caused by ammonia in the brain. That's it. It does not prolong life. Someone correct me, please, if I'm wrong. My thought is that if he doesn't take it, he could still linger on for months and be even more difficult to care for than he is now.
However, I do agree with what everyone else said. If you can do no more for your dad, you've already gone above and beyond, considering the history of the relationship.
Hugs,
Connie