The only cost involved would be him having to buy his own plan. He wouldn't even need to change anything with the registrar, since the nameservers are the same.
Now that I think about it though, I'm not sure how the DH system would handle him adding the domain while it's still on your account. If there's a check to stop this from happening, I'd assume that it would be system-wide and not just per server.
There should be a way that lets it happen, without you dumping it, then him adding it--but it might have to be done by support.
On the other hand, since the nameservers would be the same, if you deleted it, and he added it right away, there shouldn't be much of a downtime. The "Oops, I didn't see that coming" to this method could be that you delete it, but it hangs in the system, then doesn't let him add it right away.
Unless someone else has done this here and speaks up, I'd probably run it by support and see what they recommend. I've only ever had one account at a time, and just added users, so I never had to push anything to another account.
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