You need lighter advance springs. The timing is lower when cranking, but once running, the springs are light enough to allow mechanical advance to kick in at idle. Since mechanical is already operating at idle, there is less available at higher rpm, so your total won't be so high.
He'll never get a stable idle if he does that, unless the advance is all in at idle (which is no good for the street). If the advance is already operating, more timing = faster idle, more timing... put it in gear, slower idle, less timing, slower idle, etc.
