- delete the chat account
- notify the SE team so they can reattach or clean it up
- leave it as is: it's a normal situation
For the time being, please leave them as is if possible (maybe ping me about them). Better handling of these cases is part of the planned improvements I mention here, so having real examples handy is nice.
In cases like this which have to be handled, because the persion is actually still active, and it's just that one site where their user was deleted (and which also was the parent user), there's not much you can do; this has to be fixed by one of us. Should be rare enough, and won't be an issue for much longer anyway.