Is it wise to turn off the iPad completely if I don't use it during the day? A friend of mine has an iPad that she uses only once a day for about a half hour.

I told her to turn it off completely in between uses to conserve energy - no point having the device run in standby for 23.5 hours a day. (So no standby, but a real "swipe to turn off" shutdown.)

I'm not completely sure whether this is correct, though. Is this the optimal way to go in terms of optimal battery lifespan? Battery life (i.e. the time until she has to recharge it again) is secondary, we just want to preserve the battery for as long as possible.

Apple's iPad battery tips don't give any hints, nor does what appears to be the reference question here on Apple.SE.
The total life of the iPad's battery is going to be affected by the temperature you keep the battery at and the number of discharge-charge cycles it goes through.

Turning off the iPad will cause the battery to drain more slowly than leaving it in standby mode. But the battery will still be draining when the device is "powered off" as there's circuitry enabled to detect you pressing the power button to turn it back on.

I'm not sure how long an iPad that's off will hold its charge vs. one that is simply sitting in standby mode. I can't imagine the marginal gain in battery life is worth the hassle of waiting for it to power up every time you want to use it.