Charlotte, this sounds so awful! 😟 It sounds like someone you care about is slowly losing someone they care about, but there are maybe not enough specifics to advise?
My general rule is to insist on the truth, especially if major decisions need to be made and multiple people are affected. That said, some people are simply not capable of handling it — may lose their grip and/or push you away. That doesn’t really change the necessities, but you might have to look for ways to work around that person.
I don’t know that I’m being much of a help, but I’m here if you want to discuss more or feel free to PM.