Diet coke has phosphoric acid which causes erosion of the enamel.
It's not the phosphoric acid, its the carbonation. Plain old carbonated water is acidic as well which can erode the teeth. That being said your saliva can repair the teeth. The current wisdom is don't drink solely acidic drinks and wait between acidic food/drink and teeth brushing.
Or chew gum with xylitol (It's actually natural birch sugar) that will heighten the PH level and kill germs.