The way I suggested would be completely reversible if you kept the stock frame and sheet metal. All he would be doing is pulling the engine and putting it into another frame. Best way really since oil tank frames work much better for a "bobbed" bike or a chopper. Depending how he goes about it it isn't expensive either.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!