Well, I have had a cable box that was installed outside fail after 5 years in the humidity. When the customer tried to turn it back in - because it was so corroded & nasty - they charged them $300-$400 for the box (can't remember the exact price).
Someone has to pay for the destroyed box eventually. Whether the cable company eats it or they charge the customer for it, is anyone's guess.

I would let the customer know, since it is not their box and is on lease from the cable company, if/when it fails they may have to pay for it.