It’s hard to say definitively what will happen.
Your mil will be assessed before discharge as to whether she can go home or not. This will depend on things like whether her physical needs can be met (with carers, equipment, adaptations at home) or not, whether spouse / family members are willing / able to provide care (organising medications and ensuring they are taken, providing food / drink etc, keeping them safe / fed / clean/ entertained etc) or not, and her medical condition whether she requires ongoing nursing care or not For my MIL she went into hospital with Parkinson’s, osteoporosis, dementia already diagnosed - but it was severe dehydration / infection / delirium that actually got her hospitalised. Her already poor mobility deteriorated greatly after a few weeks in hospital and despite the efforts of the physios to get her walking etc, it didn’t happen. She went straight to a nursing home.
However, be warned that if social services are told (by FIL for example) that he can / will care for her at home, they will likely jump on this. My Own FIL had to be coached to state clearly that he could not provide the care that MIL needed and that it would not be safe for her to come home, even with multiple carer visits and equipment etc. if he hadn’t done this, she would likely have been sent home - incontinent, unable to walk, completely confused - to be cared for by a well meaning but incompetent carer, and mY SIL running herself ragged to fill in the gaps.
A as social worker will be assigned at the hospital. Are you involved OP? One issue we had was that FIL was the main contact for the hospital and it was hard get info from him and to stop him making unrealistic promises 🙄.
Whether or not she goes to a nh or gets carers at home, a financial assessment will be carried out to ascertain how it will funded. Age U.K. has great info - remember that the situation varies btw England and Scotland. It depends on what assets (property, debts, savings, income) your MIL has.