On the issue of placing MIL partner in a care home without him having notice, this won't actually be true. You can only be placed in a care home with full consent of the person, or if you have lost mental capacity and a best interest meeting has taken place. At the meeting, which will have involved a social worker, and his family who held POA, his care needs will have been taken into consideration and if he required 24 hr care, a care home placement would have been found for him. No care home would have taken him in if this procedure wasn't followed, as they do an assessment themselves to check if they can provide the right level of care, and if he had capacity and refused they would not have offered him a placement,
It is a dreadful situation for MIL to be in, but as his non live in partner only, the family, have the legal if not moral authority to withhold this information from her.
People with dementia often accuse others ( without basis) of some dreadful things as it progresses accross the brain, and they lose function. It may be that he has accused MIL of mistreatment or financial misconduct, and the family have taken this at face value and believe they are protecting him from her.
Perhaps you can ring around care homes in the area to find out where he has been placed? If so, she may be able to visit him as a friend. Be aware though, if she has been accused of anything by him, the family have the right to refuse her permission and the care home will be obliged to block them.