Said passenger needs medical help ASAP. Go to the nearest destination (in *time* terms) that meets all the relevent regs, make sure ATC are aware of the medical emergency and hope he's OK in the end.
If you are 10 minutes into a 40 minute flight, you are only 25% of the way through it. Will it really be faster to continue? If I was a passenger returning for a medical emergency (if that was the fastest way of providing qualified & equipped assistance) would be comforting, as it's what I would want for myself if I was ever so unlucky.