How do you set a feerate to zero by cli in c-lightning to prepare for a re-balance?

See if this is helpful for you: https://lightning.readthedocs.io/lightning-setchannel.7.html?highlight=fees#

Thanks for that I eventually found that and it allowed me to set feerate to zero

Excellent!

I heard someone had a bad actor connect to this someone' node and drained their funds. They did it when he changed his fees to zero for a rebalance. Can you explain how that could happen? Is it a vulnerability to have zero fees on your channels?

No. They can’t steal funds. Drain means they drained a full channel and filled up an empty channel by forwarding without paying fees for this. It’s not a vulnerability, but a feature. You can set your fees to anything you like.