In my most recent game my opponent was playing with Luke and Leia as his commanders. At one point, he used one of Leia's command cards, but extended its effect to range 4 based on Luke's Battle Meditation. This seemed fishy to me. My logic being that Leia does not have Battle Meditation and therefore her cards should be limited to range 3. After all, once a commander is defeated, their cards are no longer available. The rules say that a commander cannot use an operative's cards. It seems that there is plenty of precident that a Commander's abilities are in separate silos, so to speak. But, I couldn't find anything that specifically disallows such shenanigans as this in the rules.

So, what do you guys think? Can you use Leia's command card at Luke's range? Or is she limited to her own range?

Your friend is wrong. Page 21, under Command Phase, step 3 Issue Orders, first dot point.

Leia must be the nominated commander that turn if you are using her card, so Luke's upgrades are irrelevant.

»» If a commander-specific or operative-specific

command card is played, the player must nominate that
commander or operative.