I can understand why some people are prickly about this question.
I've been sick now for almost two and a half years following an event that for most people would be a non-serious incident which they recover from in twenty-four hours. It was a completely freak accident, and it's left me with a seriously reduced quality of life, unable to do some many normal things I took for granted - like walking any distance, driving, drinking, etc etc.
I don't think my employer has income protection (?) so this will very much depend on your employer, but once you run out of paid sick leave, I think some people end up on unpaid leave (or SSP) for a good while. Luckily I've been able to go back to work, but I returned way too soon, partly because I felt under pressure to try.
People tend to expect you to just get better in a nice, neat, linear way. The reality is that many conditions involve ups and downs, setbacks due to other life stuff (I'm currently stuck off work because an ordinary viral bug has left me now unable to look at a screen for more than a few minutes without horrible pain and nausea). And what people don't seem to realise is that we can't predict the future. We don't get reliable prognoses on which to make decisions, we might even have reduced capacity to deal with making decisions (or the life admin around it), and being chronically ill is really bloody expensive. So of course we're not going to just quit! I never would have thought I'd still be in this state two years ago, and the doctors sure as hell didn't - A&E thought it was so trivial they barely wanted to even see me! Nor do I know now if I'll be able to work next week, next month, if it'll improve, deteriorate further... A surprising amount of medical science sadly seems to involve just leaving the patient on their own to see what happens.
As others have said, your issue is with management, not the person in question. Framing it as you did may make it seem like the latter (not to mention the other person who referred to sick leave 'stringing along' their employer). I appreciate you might not have meant that.
I'd suggest you just talk to your employer about workload. It doesn't need to be directly about the person in question at all - though I'm surprised they haven't changed anything after this long. Raising workload, and flagging the risks and things that won't get progressed without a change, is perfectly reasonable.
Needless to say, all of us who are chronically ill would bloody love to be well enough to work reliably. We're not having fun lying at home. Not being able to work usually means not being able to do all the other normal life things reliably too - like leave the house, drive, go for a walk, go to the cinema, have a pint, cook nice meals, etc. etc. It's bloody miserable and it's a shame - whether or not the OP meant this - that we've become such a society that bashes the sick and disabled without blinking.