Given that Mr. Hendricks seems happy and content in his current life, it would seem that preserving this state of being for him would be the most beneficent course of action.
We might get a sense of how Mr. Hendricks understood his best interests if his advance directive or previous conversations described his reasons for wanting to refuse any curative treatments at this stage in his disease progression.
Our understandings of what constitutes beneficent action generally as well as of our own best interest shift throughout the life course. For Mr. Hendricks, the course of action that is most aligned with beneficence might be to respect his advance directive if it is anticipated that curative treatments might cause significant reductions to his happiness, function, and overall wellbeing. There are strong arguments for focusing on quality of life as opposed to strictly quantity of life, but there is no consensus across the board that one of these is more strongly aligned with beneficence or best interests.