As Hal said 1st check condition of battery & battery connections and weather eng can be rotated by hand.
The oil light being on when ign sw in on/run position is normal due to fact eng has a pressure sw on oil filter adapter & eng has an oil pump,light will go out after eng started if lube system working ok.Light has no effect on cranking system.
IF battery/battery connections prove not to be the problem & eng turns free,then take a jumper cable directly from battery pos(+)to battery terminal on starter,if starter spins eng then starter motor ok,check battery connections at starter motor & sol for starter that they are clean/tight,if connections ok then connect a voltmeter/12v test light to coil terminal on starter sol,when holding ign sw in start position check for voltage at coil terminal,if you get a voltage reading/light & starter doesn't crank eng replace sol,if no voltage at coil terminal work back thru wiring system from sol with a meter/test light & check brake sw,pto sw & ign sw operation.
Tell your neighbor to do a goggle search for cub cadet 2166+wiring diagram,there are several posted online for tractor wiring to help troubleshoot problem.
IF your neighbor doesn't have the electrical knowledge/skills and tools to troubleshoot problem,tell him to leave it for pros at local mower shop.