So my 16+ year old kitty is going through something similar. We have found a thyroid issue to explain the weight loss. she also has been staying in a different room much more lately, she is also spending more time standing on the heat exhausts.

Its my understanding that as cats age, they need to be in a better lit environment, staying warmer, and on softer surfaces. I've adjusted those things and it's made a pretty decent difference.

Hope this helps and I'm sorry you're getting to that point where you're questioning on how best to care for your friend.