You folks are finding problem where none exist. I'm not speaking of being shabby, but of legal or ethical issues involved.

The prints were sent to him with no expectation or claim of confidentiality. The implied copyright would bar him from commercializing them, but not from sharing or even forwarding the original to a third party. From the copyright standpoint that would be equivalent to sharing a book. The information that might be gleaned from reading the print isn't a trade secret, since anyone could simply find a frame and take some measurements.

If the company that sent the print intended to protect any trade secrets they had a simple remedy - don't mail a print. But they did and he can use it as he sees fit. That doesn't make him a nice guy but that's the way the cookie crumbles.

If it is a print of a stock frame, he should cut off the name, and send a copy along saying it's a print drawn up for a custom frame. The Chinese factory would have no reason to think that it was anything special and wouldn't be especially inclined to copy it for a production model, thereby offering some insulation to the original maker.

As a practical matter it's impossible for a manufacturer to totally insulate himself against being copied. Even patents aren't bulletproof protection. Designs are copied all the time, so what we're talking about when it comes down to it is a piece of paper, sent with no claim of privacy or conditions on it's use, and as such he can use it as he sees fit, subject to his own sense of propriety.