Tom-
A few weeks ago my MSD 6AL failed, and one symptom before it quit completely, was a wildly bouncing tach. This MSD web page tells how to test the box. It's really simple, no meters needed:
MSD Ignition
The test takes the distributor out of the circuit and should confirm that the MSD box and coil are ok or not. Elsewhere on the MSD site there are instructions for testing the resistance across the magnetic pickup in the distributor (assuming this is the kind of dist. your friend has). Good luck.
Jerry