My boyfriend has a friend that claims his mother played the part of Elly May for a couple of episodes? was there ever anyone else cast as elly may?

No. Donna Douglas was the only actress to play Elly May Clampett (at least
until Erika Eleniak essayed the role in that dreadful theatrical movie version).

Eva Gabor played Lisa Douglas, who played Elly May in the Green Acres episode where they put on a play of the Beverly Hillbilles.

Maybe the friend's mother was Donna Douglas' stand-in?

No. Donna Douglas was the only actress to play Elly May Clampett (at least
until Erika Eleniak essayed the role in that dreadful theatrical movie version).
that theatrical movie was HORRIBLE!!!

(although, Jim Varney was ok as Jed, not good but not bad, either. But that was the only half-good thing about it)