You don’t need cadence (its nice to have though), you need a speed sensor and a supported trainer, unless he gets a smart trainer, then no need for the speed sensor. Or a power meter.
He would need a power meter then, if one will fit on that spin bike. Pedal based is probably the easiest, but they are hundreds of dollars. Might be cheaper to get a used road bike and a dumb trainer with a speed sensor.
Would a Speed / Cadence sensor be of some use? I presume wattage can not be “reverse” calculated from one of those in Zwift? Assuming that could even be mounted some how on the fly wheel…
Was just reading over power and seems pedals are the way to go, but as you said, they get pricey!