If she does leave the room, she really can't go anywhere unless you are in port. My mum has beginning symptoms and I took her on a cruise this past Spring. She forgot her cabin number a few times and the purser and stewards assisted her back to the room. In new situations, she simply cannot cope anymore and gets lost very easily. You are lucky she will have a roommate. My DH couldn't come at the last minute so my cabin was on the starboard side while my Mum's was directly across the ship on the port side. That caused a few problems, of course.
It's not a bad idea to make sure your MIL wears Depends (even if she is not incontinent, she may get lost on the way to a restroom) and her S&S card on a lanyard around her neck and a medic alert bracelet. If she loses her card, the purser will issue a new one and cancel out the old one so it can't be misused.
I had a huge problem monitoring my Mum's sugar intake. She would sneak up to the Lido for cake and pie, not good foods for a diabetic. If the ship's personnel are aware of the situation, they can be a big help. She was embarrassed when she forgot to wear the Depends and the inevitable happened but the steward must have been very experienced with the situation and whisked away the soiled clothes very discreetly and returned them the next day.
Do not let her carry her handbag around. Mum left hers on the Lido Deck on a table outside. About 45 minutes later we were retracing our steps and there the purse was, in plain view, undisturbed and intact. I breathed a silent "Thank you" to all our fellow sailors who displayed such good character. Losing her ID, insurance card, credit cards, etc., would have been a real problem.
I worried more in port but since we went to Canada, everyone spoke English so it helped her feel more comfortable in the strange surroundings. She would forget to button up her coat and her gloves and scarf so I just made sure the gloves were fastened to the sleeves of her coat with that little clip on device they sell just for that purpose. She still managed to lose them somewhere.
You can find a wonderful device at Bed, Bath and Beyond. It is a magnetized device. After the cabin door is closed, her roommate attaches it with double-faced tape (included). If the door is opened the magnetic field is interrupted and an insistent beeping starts. There is a little switch to turn it off when you leave the cabin in the daytime. If you have time, please e-mail the address and I would be happy to send you the one I have. Might need a battery.
You have my sympathy. It is not an easy disease for a family to cope with. I won't be able to take my Mum on a cruise again but at least she experienced it once - and somewhere in her subconscious I know your MIL appreciates everything you do for her.
Suzie
P.S. After we got back from that cruise, I immediately booked a short one on the Elation for my DH and me - just to recover!!! You might consider a vacation after the vacation, too.


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1/09-Carnival Pride-Mex. Riviera-Cabin 8239
5/09-Carnival Splendor-Canada-Cabin 2465
6/09-Carnival Elation-Baja-Cabin E56
11/09-Carnival Spirit-Exotic Mex.Riviera-Cabin 7303
1/10-Carnival Splendor-Mex. Riviera-Cabin 2***
8/10-Carnival Spirit-Alaska-Cabin 6***