Yes, you are absolutely liable for what your dog did.

Without corrective lenses I'd be almost legally blind. I have glasses and contacts. Without insurance my glasses were approaching 1k, but I also have the special thinned out material so I don't look like I'm wearing coke bottles, coatings for computer use etc.  

Go with him to the optometrist and get a quote for repair/replacement. Might be able to use the lenses in a new frame, for example. And then take responsibility for what your ill trained dog did. Be a better friend and pet owner.