My son has type 1 and has been in 3 hospitals. Diabetes + mental hospitals is an absolute nightmare!
The biggest problem we experienced was finding a hospital that would even take him. They do not want the liability, because they don't usually have medical staff on site in case there's an emergency.
He was sectioned in the ER 4 times. One of the times, they could not find him a place and ended up sending him home because the hold expired.
Two of the hospitals that did accept him were fine. He wasn't allowed to test his own sugar or give himself shots. He had an insulin pump, but it was confiscated on admission. Still, they did a good job.
The third one, though... OMFG it was the worst. They mismanaged him every way they could. Skipped shots, late testing, very high sugars. And they had the nerve to blame it on him! I got several calls from him a day saying he was terrified they were going to kill him. We ultimately had to threaten lawsuits and force them to discharge him so we could get him into a better place. I don't know how fearing for your life at the hands of the staff is conducive to good mental health.