Tell your friend that if he can't be bothered to see a doctor, not to worry. Just keep running and when he collapses in the road at some point, some nice men in an ambulance will come out and take him to the hospital emergency room. He won't have to lift a finger.

On a somewhat more serious note, many of us (including me), tend to play the denial card when we get symptoms of serious health problems. Trust me on this, early treatment and diagnosis of any disease beats the hell out of waiting until it forces you into action.