>>1250824"be careful" is sad alot and most the time people just ignore it.I don't agree with you. If someone hands something to me and says "be careful" then I make sure I'm handle it carefully. Why? Not only is my mind now aware of my handling of the object (before I might be on autopilot), my ego would now be involved - dropping something after someone says "be careful with that" would make you seem extremely foolish.
OP, you should have said "be careful, it's very expensive" and I like the suggestion about making her wear the neck strap (just because the camera costs so much - I wouldn't have her wear a neckstrap for my Rebel). Technically you could sue if it was damaged due to her negligence, but just treat this as a learning experience and move on.