Before you do anything, ask him what it was he used. Pat M gave you some good advice, use Liquid Wrench to 'rinse' away whatever it was he used. And then use sewing machine oil.
If it was WD-40, I wouldn't worry too much. WD-40 evaporates away anyway. It's does a temporary job at lubricating, which is why you don't find it listed as recommended in manuals for sewing machines, lawn mowers, tools, machinery of any kind, etc. There are properties in it that retard rust, which is why I coat my gardening tools with WD-40 before storing them away before winter. But it's worthless as a lubricant, it evaporates within a month!