Okay... there may be 6 people playing the game. If NONE of them choose to be a healer, or take any healing-inclined items or feats along with them, then they have set themselves up for failure. It is not your job to provide them with a healer. It IS your job to provide them with a plot hook, and a story.

Kindly recommend that someone do something to help with the lack of healing in the party, but if they choose to shoot down this recommendation, then let them go on without healing. They will soon learn that having some kind of healer is a good idea.