Everything FriedGreen says about balancer and how hard it is to detect is also true for motor mounts. If one is "sheared", the weight of engine will keep it looking good to the eye. Some component is broken and is reaching a certain natural frequency, then passing back out of that frequency (harmonic balancer, motor mount, maybe even a misfire caused shake) Hope your buddy did not just do some kind of work on engine like put in an externally balanced component or some other no joy.
