It's still early days. Try not to panic.
There could be a number of reasons he seems to have mentally declined. Stroke can affect cognitive processing, memory, speech (which includes understanding, getting the words out, etc.). Delirium could be a major factor at the moment. It's really common in older people.
In the first instance, see if you can speak to the Occupational Therapist on the ward. Ask them to clarify exactly how the stroke has affected your Dad and what input he has had (i.e. occupational therapy, speech and language therapy, physio) and what he needs going forward. This should at least give you a bit of clarity and, depending on what's said and his deficits, you may want to speak to the speech and language therapist and the physio.
In terms of your Mum, as harsh as it sounds, you need to tell her that she's got to pull herself together. This isn't the time for her to fall apart. She needs to be strong for all of you especially your Dad. She is probably scared but at the moment she is making it all about her which it's not. Her inability to cope could be the very reason that he is having to go into a home. Give her a notepad and a pen and tell her to write down anything that happens when she is there (i.e. time, name, who they are, what they said, what happens next, etc). If she misses any details, tell her to ask them again so she can capture it/update the rest of the family. Ward staff are generally very understanding.
The Stroke Association has a really good website full of useful information. You can also contact them if you want further help and advice:-
www.stroke.org.uk/
Hopefully the dust will settle/he will improve when he gets out of hospital. At the moment, although it may not be evident, he is still recovering and that can take weeks/months/years.
Wishing your Dad all the best. Please don't give up hope. x