My husband keeps plugging up toilets. any advice?

2 years ago we lived in assisted living and my husband kept plugging up the toilet when he had a bowel movement. We moved back home and for the next year he only plugged it up once. Last month we bought a condo and the 2 toilets have constantly been plugged up by him. The plumber has been out several times in the last month. He unplugs them with a plumbers snake and checked the outside drain. No problem. I bought two expensive toilets. Same problem! He doesnt seem constipated. He is on constipating pain medicine but takes two senacot pills for constipation plus a few dried prunes. He is a small man. During the night he plugged it up withdiarrhea. I don't plug it up. Ive tried flushing it for him and made sure he isnt using too much paper. One of the toilets is always the first to plug and then the other one follows. Any ideas??

I am today finding black (not redblack) pieces on the floor after my husbands diarrhea. Some are tiny, some about half the size of a dime. They are stuck to the floor very tight and the dirt will not come off the sides of the bowl. Does anyone have experience with this? Thank you for your help.