Getting a lot of miles out of one DVD might not be a problem, but that kind of forgetting would seem to be. It points to other memory issues that can greatly interfere with life.
During his last three years, my father-in-law could make do with one conversational topic, endlessly recycled. It might have been economical for him, but it drove other people nuts to hear the same comments a thousand times, sometimes only a few minutes apart. He couldn’t follow the plot of a movie or remember the action during a football game while the game was still on.
He also forgot who his sons were during the time it took them to leave the room and come back. As a result he thought they were intruders and kept screaming at them to get out and quit stealing his stuff, even when they were bringing the groceries in or bringing him a meal. This made life pretty rough on my husband and his brother.
On the other hand, it was probably a mercy that he forgot he had cancer as soon as they left the oncologist’s office.
I’m guessing that your mother-in-law must be in her eighties or late seventies. Has she been evaluated by a professional?