I know the feeling, having lost a tree at a show a couple of years ago. There are electronic security systems available, but not sure how effective that is. Usually sounds an alarm as the person is running off with the tree.
I have a digital photograph of each tree on file, and can pass that around the area clubs in case someone shows up at a sale or auction.

That stinks. I'm going to start growing only trees that have large sharp thorns. I'll train my dog to attack any who approach them too. :-)

I hope your friend is able to recover his beloved trees.

Not even a dog can stop a person who is intent. But 2 or 3 dogs will discourage the most foolhardy unless there are more than one person or that person is familar to the dogs.
The best bet is a sound security system based upon appropriate fencing. There is that steel fencing that are tall post that end in spikes and are bolted onto a concrete base. Check with a reliable fencing outfit in your town for ideas. It may cost but it will make an casual or intentional thief take a second thought about the trouble to steal a plant. Also I would hope that there is an international community website where such plants that are stolen are imaged and all bonsai clubs throughout the world can access the site to check out plants being offered for sale. This would really put a bite in the after-market of hot merchandise unless the thief is collecting for themselves.
Also there are relatively inexpensive camera systems that can operate off your computer and with the right kind of sensor arrays can be activated to image any disturbance day or night-again it is all a matter of money.

If you are strapped then welcome to my world. I have had to deal with such losses and the plants were only starters that I was just getting going. I have also had to deal with cruds that destroyed what they could not steal and also knifed my dog in the process. What I wanted to do was to set up a machine gun with a sensor array to sock it to any creep that ventured into my yard but then I would hate to come home and find the meter reader all shot full of holes and the police waiting for me!!!!!!! OOOPPPPSSSSs!!