I can’t comment on the co-op because I have no experience. However, I am almost 70 years old and have been widowed for 13 years. I currently own a condo. I promise you that by the time I am 87, I will NOT want to OWN anything - home, condo or co-op. Even now, I think about moving into a nice apartment. As a condo owner, I am still responsible for everything on the inside and some things outside. At age 87, when something goes wrong, I just want to call someone and know they will handle it.
Have you considered a 55 and over senior apartment complex or even Independent Living for her? What does she think?
Good luck! I hope it all works for your family.