We didn't do that one but are experienced snorklers, your wife will hurt the coral much more than it could hurt her except in the case of fire coral. If you see bright red/orange coral STAY AWAY it will "burn" you. It is important to not touch any coral as it is a living organism and you will kill parts of it if you come in contact with it.
A great snorkle trip on the Eastern is the Trunk Bay snorkle, it was just beautiful with large schools of fish you can swim through!