Travel insurance is your friend. If the insurance is purchased within 14 days of the deposit/air purchase it should cover pre existing conditions-which he does have!
Chemo affects everyone differently with regards to side effects. My niece did better than most but even she would never have been able to fly the day after any of her chemo treatments.
So very sorry for your friends diagnosis and I hope he will allow friends and family to pamper and spoil him.
You are a very good friend.
However, the very recent news about so many Americans dying while on holiday in the DR makes me wonder why he wants to go there? I don't think the cause of these deaths has been determined yet.