I have sold my business and the new owner now owns the QuickBooks online with all clients - How do I transfer 1 client to their own account?
Do you mean transfer the client from the old Company/QuickBooks Online account to a NEW Company/QBO account? If so....
You can't delete Customers (or really any acct.) in QBs that has any activity in it. So that means you can not completely remove the client from your old/sold company. But you can make that client Inactive in the old QBO. (I trust there is no outstanding balance. If there is, you may have to wait and have the new owner make the client Inactive after the client account has a $0 balance.)
If you want historical data on the client, you can create a full report and export the info to Excel. I'd also print a PDF of the report to just to have it.
In your new QBO account, just add them as a New Client and move forward from there.
NOTE: Although there are ways to export data from one company and import it into another, you probably do not want to do that. All the Income and Expenses related to that Client are part of the old company. You don't want to just drop those numbers into your new QBO company.