What is the best chair to prevent slipping and shearing?

My 89 year-old mom has end stage Alzheimer's and she slouches a great deal and tends to slide down the chair. It is terrible for her buttocks, even while using a ROHO cushion. I use a lift chair because she adamantly refuses to get up and that is the only way I can get her up and moving. I was thinking of getting a geri chair, but would that prevent her from slipping down? Can anybody recommend a good chair for her? I don't mind paying if it helps her. I know Medicare will provide a gerichair but I do not know if that's good for her condition. Keep in mind it's very hard to get her up -- that's why I use a lift chair.

Does anybody have any experience with this and how do they handle it? Thank you.