He should come out pushing a wheelchair.
That's a tempting thought, but I think if Hillary has difficulty at the debate - coughing, trouble standing, etc., that last thing Donald Trump should do is be sarcastic, which would only generate sympathy for Hillary. It would give the media the opportunity to turn the focus away from Hillary's condition and make her the victim of Donald Trump's "mean spirited, insensitive comments." Instead, he should express concern, ask her if she's okay, and suggest to the moderator that maybe they should take a break for a few minutes. If Gary Johnson is on the debate stage, he will not want to look like he doesn't care, and would probably join Trump in expressing his concern. Trump will not leave himself open to criticism, and all of the attention would shift to Hillary, raising still more questions about her being physically able to assume the duties of president.