Most likely he screwed the trail up, making the bike unstable.
Tell him to check it using a rake and trail calculator
This one has more info
http://www.rbracing-rsr.com/rakeandtrail.html
But this one is a bit easier to use.
http://www.goldammercycle.com/hispeed/rakeandtrail.php
Don't tell me its cruiser/chopper shit. Its all about geometry.
Further for balance issues... go dyna beads and STU
http://lowdownchopshop.com/shopping-cart/14.html