The current year, plus two prior years are efile-able.  So your 'client' is correct.

And there is nothing different about the procedure. Prepare the return, check the results for accuracy, then click on the efile button.

2018 had a little update quirk earlier in the season, you may need to use REFRESH Updates in order to get the Efile ability to work for 2018.

How do we even prepare a 2018 return using Pro Series 2020?

I seem to only be able to open a 2020 return and I have new clients needing 2018, 2019, and then 2020 filings.

You need the 2018 and 2019 programs. 

Thanks.  I got 2019 downloading now, but there's a gap (2018-2013) is missing from "prior years".

I have one client that needs the full (1065, K-1, 1040) treatment for 2018 though.