I don't think this is possible.
In order for your friend to host the sub-domain, they would first need to add the parent domain to the hosting system to prevent the system from throwing an error when adding the sub-domain. However, they will not be able to add the parent domain if it has already been added under your account.
Edit: One option may be to have your friend setup an entirely different sub-domain under his account, then set the sub-domain under your account to re-direct to his sub-domain, if that makes any sense.
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