How to deal with an angry, irrational woman when telling her that she has to remain at home with us?

We are so overwhelmed right now!
My MIL is 73 and had a quad bypass just over 3 weeks ago. The hospital released her to a rehab facility. She has been showing signs of early dementia before her heart attack and surgery yet has not received an "official" diagnosis from a neurologist etc.

She has shown huge increase in confusion and memory issues since the surgery. The doctors warned us ahead of time that this could very well happen so it was not a surprise.

Our dilemma - She has been living independently with an 85 yr old male "friend" with whom she has had a very volatile relationship with for 25 years. He moved in with her 4 years ago after he had 3 stents put in so that she could take care of him. He has his own house 35 miles away and his own vehicles. He pays none of the monthly bills but on occasion will purchase something like a "new" washing machine etc. Right now he has been living in her house and driving her car. He is verbally and mentally abusive to her and has been shown to be physically abusive to her in the past. No amount of logic applied before dementia started would convince her to break off the relationship. She is determined to return to her own home even though it is not a safe environment (she lights her gas stove and walks off, stairs she can no longer manage, expired foods which are WAY to old and nasty for consumption, small spaces difficult to manage her walker, memory causing troubles with her ability to maintain her sternal precautions, her "friend" being an alcoholic abuser, previous history of falling and the list goes on and on....).
We have dealt daily with the temper tantrums about staying in the rehab until the doctor would sign a release and have had to spend nights and days with her there simply because she didn't want to be there and the facility was not a secured environment due to the hospital error in placing her there.

We are submitting for temporary guardianship as I type. My husband is the only sibling of the two who want anything to do with the situation.

The doctors have said that she can no longer drive or live by herself and that she is to remain home-bound for at least another 4 weeks during healing. Even though he knew the doctors had said no car rides, her "friend" took her for a joyride 4 days (11 days after surgery) after she was in the rehab without telling anyone where they were going. During the 5 hours they were gone we filed a welfare check with the police and checked every place we could think of to no avail. She thinks that her "friend" is capable of taking care of her.
She agreed for 4 days straight that she would come and stay with us at least during her healing time of the next 3-5 months. We have spent almost $9k making our home safe and as comfortable as we can for her.
She has now agreed only to stay with us a week...

She refuses to follow the doctors orders on anything that keeps her from doing what SHE wants to do WHEN she wants to do it so using the reasoning of "the doctor says..." is not an option for convincing her to stay longer for her own good. Her entire focus is worry about the "friend" leaving her. She believes that if she goes home then everything will return to "normal". Unfortunately he won't drop out of the picture on his own, seems to believe that she will be "just fine" once she is back home (doing everything for him while he is at the bar all day, most days and of course she is not going to tell him to leave permanently!