The nut on top will help but the problem lies with the nuts under the triple clamp and / or the bearings. Support the bike with jackstands and the front off the ground, then take the clamp off and check that the nuts underneath are tight. Grab the end of the tree and see if there's free play, if there is and the nuts are tight your bearings are done... although I've never had that happen with any of my racebikes regardless of mileage so my guess is that you should stop doing stoppies if you really want to cure it.
You probably shouldn't tell him to just tighten it. since the Torque spec for the Spanner nut is probably around 4ftlbs. Doing endo's are very hard on your steering head bearings and i would guess they are probably shot. Take it to a shop and have those bearings checked/replaced. if you just crank down on the nut and over tighten them you will have worse problems down the road