Do you have third party authorization? If so you should be able to just call Practitioner Services and have the refund mailed to the client. https://www.irs.gov/tax-professionals/practitioner-priority-service-r

In the olden days, before this little pandemic thing, the taxpayer could call 800 829-1040 and ask for it to be refunded. 

Life is a little different now, but I am pretty sure that a 1040X is not the solution. It would be a paper form that MIGHT be processed before the client needs to file for 2020, but .....