Of course you did the right thing taking away expired meds. Most just lose their oomph, but others can become dangerous. Your MIL may think she needs the laxatives because some of her other meds and/or diet may make her irregular. The dr would probably recommend more fiber in her diet. Also she may be of the generation where castor oil was the cure of all ailments ... tastes awful, but cleans you out. If she shouldn't be on a laxative, maybe the doctor could suggest a safe placebo, like a tic-tac.
My aunt's husband lived into his 90's. When he died, my cousins and I had the task of going through his things. We found dozens of tubes of hemorrhoid ointment and tissues in all his clothing pockets. Who knew?
Good luck with your MIL. Please let us know how it goes.