What to do his best friend is dieing ?

The guy that I have been seeing has a best friend who is dieing of a lung disease, his friend barely made it through last winter, now since summer is ending the guy ive been seeing is acting very weird. He has cried to me and told me how he fears of losing his best friend, and now he is cutting everyone he cares about out. His new motto is eff it. What can I do? Hes pushed me away, I want to be there for him but dont know what to really do or say? His mother and I are worried about him.