He has verified voltage at the starter solenoid and has tried jumping the small to big terminal. This eliminates everything before the starter in the starting system.

The only thing not conclusive here is ground to the starter, which is pretty rare since it's bolted to the block, just like the negative terminal from the battery.

Before condemning the starter. It may be the big starter wire. If it's corroded or has broken strands in it. There will be 12 volts, enough to make a multi meter read 12v or a brighten up a test light, but not enough current to engage a solenoid and spin the armature.

Or ya, the starter is bad.