depends at what point in the game he gets them. someone on TL made a good post about a week back regarding all things PvZ.
if you already have robo tech, don't make any colossi but you often want a few observers to be able to tell where the mutas are.
other than that, get as many cannons as you need to survive while getting blink and a templar archives asap with storm. storm>>>>>archons.
if he really is going mass mass muta, just make a million cannons everywhere- remember that he can always force a base trade if he wants, so make it such that it's not beneficial for him to do so.
also remember that mutas are often accompanied with ling/bling, so throwing in a colossus or two once you've got sufficient templar is usually a good idea.