road chief

Someone who watches another person while they are tripping on a hallucinogen to make sure they don't hurt themselves or someone else, and keep them calm if they start to have a bum trip. They usually also help the person who is drugged to interpret the experience.

The term was originally used by the Native American Church for the shaman who is running a ceremony, especially one involving peyote.

I agreed to act as a road chief for Mike while he was dropping acid.