Oh my!
"Dog tame"?
Tell him they are not dogs. Or mammals.
Whatever attitude he gets from the tegu, he should expect the same plus about 30 pounds MORE of it from a water. More teeth, and a real long flexible neck, much better for twisting around and biting.
Unless he keeps it really inadequately heated. If its sick and cold, it just might act like the doorstop he always wanted.
However, it will not grow big, if kept sick and cold. Hmmmm.
I'm being sarcastic here. Its what I do. However, in all seriousness, it is sad and distressing to hear of a really naive person wanting to force a wild animal to act unnatural. They are not people loving or pleasing animals, like dogs are.
Please tell your friend, if he really cared for animals in general, and monitors in particular, he'd do a lot more research, and be realistic about what a monitor is and isn't. And put aside his selfish desire to force it to be something its not. A scaly dog.
D.