Easiest way is with first time abatement.

https://www.thetaxadviser.com/issues/2013/jul/buttonow-july2013.html

2017-47 - if they filed late because of the change in due date of the return, then use that.

If the partners filed their personal returns timely, and included all of their distributive share of the partnerships income (and other requirements) then you can use Rev Proc 84-35.

https://www.bradfordtaxinstitute.com/Endnotes/Rev_Proc_84-35.pdf