Make sure it has been greased. Pull the cover off the belt/pulleys and inspect. You'll probably want to put a new set of Kevlar Gates B43 belts on anyway (check size, pretty sure it is B43 but that is from memory). Belts should be tight with about 1/2" thumb deflection.
If the flail is mounted then have him spin it up. It will shake a bit at start up but should settle down and be reasonably smooth at 2000rpm. When the motor is off, lift the flail high and inspect the rotor and all 28 mounts. Knives and clevises are replaceable/wear items but that rotor is more than half the cost of a new flail to replace so make sure it isn't damaged. The rear roller might be dinged up, no problem, but it should turn easily and not be "banana" shaped. Check to see that the main bearings on the rotor take grease (and ideally look like they have been greased regularly). The rear roller also has zerks on both sides (not so critical). There is also a zerk on top of the secondary drive shaft just near the belt cover. One more under the belt cover for the tension arm that can only be greased by removing it (once a season is adequate). Also check that that PTO drive shaft is greased and in good shape. Finally, just inspect the gearbox oil. Level is not critical so long as there is some on the dipstick. If the mower isn't mounted then make sure all the 3PT mounting hardware is present (just like on any other mower).