Spark, compression, combustible mixture.
For the last, STARTING FLUID removes all doubt (if it runs on starting fluid, but not its own gasoline . . . carburetor problem likely).
Don't know HOW he checked the valves; if he ADJUSTED them, perhaps he chose the WRONG TDC piston position (there are TWO, each 4-stroke cycle).
If the bike was running when you turned it over to your friend, chances are he did something preventing it starting now. Not much to go on enlightening diagnosis, but . . . you DO need the triumvirate of spark, compression, and combustible mixture for the engine to run. Reckon he didn't inadvertently engage the KILL SWITCH? If spark detected, guess not.