What is the ruling on the marriage of a man and a woman whose marriage contract was solemnized by a شيخ who later turned out to be a sorcerer?

There are no conditions for marriage guardianship required of the person who writes the marriage contract and dictates the offer and acceptance. The contract is not invalidated by the scribe being a wicked person (fasiq), an innovator (mubtadi’), or a disbeliever (kafir). Therefore, it does not harm your friend that the person who wrote the contract for him was a sorcerer, and he is not required to do anything about it.