I think we sometimes underestimate the value of a sincere, handwritten note - on a card, perhaps or notelet saying you are very sorry to hear of the medical news, that they are both very much in your thoughts and if there is anything you can do, such as lifts to hospital appointments, feeding the cat, letting the dog out , a welcome cuppa and a piece of cake, or a light supper, you are entirely at their disposal.
Follow up with specific help or offers- don’t necessarily wait to be asked.
They may be deluged with calls, emails etc right now, but that will die down - but true friends hang on in there when the initial fuss has receded..