One of my coworkers has gone downhill pretty bad over the last 1.5 years or so. She did a lot of drugs when she was younger and has always been a little loopy, but it's gotten to the point where she basically can't be relied on for anything and isn't even cognitively present a lot of the time, like she'll enter these trance like states where she does strange things (like pick something up and walk across the room and set it down for no reason) and has no explanation as to why she did it nor even any memory of doing it. I have also had to say the exact same sentence to her three times in a row before in order for her to give a response that even makes sense relative to what I said. She has good days and bad days, but on bad days she basically can't do anything.
I'm kind of at a loss as to what to do about it because our work is fairly intertwined, so I've been having to pick up and more and more slack to where I'm pretty much doing both of our jobs a lot of the time because she's off in LaLa Land somewhere mentally. Our boss isn't even in the same building as us, so he never really observes what's going on. I don't want to get my coworker fired, per se, but it also appears she's no longer really capable of doing this job... She's only 59.
Any ideas on how best to handle this?