Ye i actually wondered why he didnt turn into stone, perhaps they need to do that hand-sign she did when she turned the Navy crew into stone to activate the "skill" or whatever it is.
Or he's just immune because hes rubber (doesnt make much sense at all tho..)
Also, he said he's seen whatever it is on her back before, didnt he?
I'm really curious as to what that might be now -.-
Edit:
Come to think of it, Boa didnt even show the Navy fellas her back and they still turned to stone, so the need to actually see whatever is on her back doesnt seem to be required for the stone transformation to work.