They can change out the stock to left handed cast off.
I always thought that using "cast on" & "cast off" was kind of nonsensical when "left-handed cast" & "right-handed cast" would seem to make more sense to most people. But that's not shotgun nomenclature. Not only is the on/off designation not intuitive, to my mind it's backwards in that the predominant cast (RH) should be "on" & the rarer cast (LH) should be "off". But there's no logical reason I can think of why it should be one instead of the other.
But it is the other way around:
left-handed is "cast on".
For a long time, I would always have to reverify that because even though I had a 50/50 chance of getting it right, I always doubted myself because I knew my brain wanted to swap the terminology.
Thanks for posting the additional link!