Titchou is right in so many ways. First any medical condition or hospitalization can cause a decline. They do not understand what is going on and stress is not their friend. Also they function better in familiar surroundings. When they are "out of their element" you see the actual extend of the disease. You have taken away the long term memory that they function with and they are incapable of dealing with new because they have very little short term memory. The two together often appears as a very rapid and sharp decline in ability. There can also be a point where a dementia patient just takes a sharp downward decline... and it is usually associated with a medical illness, procedure, hospitalization, move, or other traumatic experience.
Delusions and paranoia are not uncommon and they can become violent when they don't know what is going on in their world. I went through this with my Mom.
As for the psych unit.... you have to believe they know what they are doing. I did a great big "HUH?" when I read the list of meds she was on. Those are some major meds to be on for that long and in high dosages. She truly does need to be detoxed!! They will take her off all the meds, start from square one and find something that will bring her back to the best she can be. If you do not have the legal papers then you don't have a say in her treatment.... especially psychiatric treatment. If it is a good facility, and it sounds like it probably is, then let them do their job.
Mom physician stopped two medications, started two others at the same time the facility tried to move Dad to another unit. Mom went off the proverbial edge. At that point we had to move them to a locked unit. Within a couple of weeks she was combative, hysterical, paranoid, aggressive, and generally out of control. I made the decision to use a local geriatric psych unit for med eval. She was on very little medication. On the way to the psych unit disaster struck in the form of incompetent ER staff and she ended up assaulting several people. She ended up in the correct unit but on an involuntary commitment. She could not be admitted against her will any other way without the necessary Medical Power of Attorney and that sister was not local. They were kind and informed me of what they were doing but I had no say in treatment. I will say that they did an excellent job! Since that visit Mom has been very content. It was a harrowing experience to go through... but in hind sight I am very appreciative of what they did for my Mom
If the psychiatrist in charge will talk to your boyfriend then he should speak with them. Not to tell them what to do but to let them tell him what they are doing and why. Perhaps that will bring understanding. It surely did for me and I am prone to quite the psychiatrist now
I do hope my experience will help a little. It's scary to go through but hopefully her outcome will be as good as Mom's has been.
PS... it has been 2 years since Mom was in the Psych Unit and she is still very content, even happy!