Domain registration is separate from domain DNS management. Does he have a hosting account somewhere where he can manage the DNS for his domain? If so, you can just point the nameservers for his domain to the nameservers for his web host.
Alternatively, you can have him set up with one of the free DNS services.
There may be a way of setting up a separate control panel user for a specific domain, but I don't have much experience with setting up separate users with limited privileges.
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