My 84 year old husband with Alzheimer's.

You might have to tell him that the vet said to put the dog in a kennel in your house for its safety. Then do it. Let the dog out for playtimes when you can supervise both of them.

It may be best to consider extra help for your husband... either an in-home aid or transitioning him into MC. The Chihuahua can't protect itself against your husband... now that's your job so do what it takes.

Next time your husband falls call 911 and consider having him go to the ER for assessment. Once there, he may go into rehab (paid for my Medicare). From there you can have him transitioned straight into a facility, if that's the best solution for you both. An 24/7 in-home aid would be very expensive, so a facility's value should be considered.

It sounds like you will have to give up one or the other for safety sake. He in MC or find a new owner for the dog