Asking for a friend - please would anyone be able to help with identifying what's wrong with this cactus and if there's any way to save it?

It lives on a windowsill in England if that makes any difference.

Hi & welcome! It's no fun to deliver bad news, but this plant doesn't look good. The part to the right is rotted, and the part in the back has a line where it looks like the bottom part may be mushy & rotted. The section to the left might still be OK. I have no idea what to do about this, but hopefully somebody who does will stop by with something helpful.