~ he asked if I could make him a Bitcoin address with my Core client, dump his paid share into it and pass on an encrypted copy of the private key. I said no worries at the time. Now a potential fork is upcoming I want to get it into his own address.
You're using a hot wallet for long-term storage, that isn't a best practice.
I don't think It's possible to disconnect/remove a specific address though.
Correct. All you can do is change the address label to something like "zzzDONOTUSE".
As an alternative, you can create a new wallet.dat before doing all this, so that his new private key is not a part of your own wallet. But it will still be a hot wallet.
Any advice would be appreciated. Cheers.
My advice would be to create a paper wallet (
bitaddress.org). Take all safety precautions: run it from an offline Linux LIVE CD, use a cheap laser printer, don't show anybody the private key and QR-code, and tell him to store it safely.
You could print several backups (more risk of exposure but less risk of losing them), and/or BIP38-encrypt them (safe against theft, but you absolutely must remember the password).
A properly made paper wallet is considered cold storage, a can't be stolen by any online hacker.
Now a potential fork is upcoming
Did you do all this before or after the Cash/Gold forks?