If the return is being rejected due to a return in that SSN already being filed, yes, you need to print, sign and mail the return in for processing.

Now if the client used the non-filer site to get the EIP/stimulus (even though it warned them NOT to do so if they had a filing requirement), then you need to follow the instructions from IRS.
https://www.irs.gov/newsroom/amended-eip-return#:~:text=You%20must%20complete%20and%20print,the%20IR....