If Hillary starts coughing, Trump should in a sympathetic manner ask her if she wants a timeout or wishes to cut the debate short.
I agree - that's exactly what he should do. Don't jump the gun if she just clears her throat or something minor like that. However, if it is clear to everyone that she's obviously in distress, he should politely ask her if she's okay, then turn to the moderator and suggest that maybe they should take a break for a few minutes. If Hillary objects, and insists on continuing, Donald Trump should say nothing further about it (he won't have to - millions watching at home will know what they saw.)