Spare/2nd X5 key no longer starts car. How to recharge key?

[TurnAround]

2003 X5 came with 2 keys. My wife and I share the car, but I drive it 90% of the time. Her key has worked all these years, until this month. We went on a trip, loaned her key to a friend so they could drive the car to keep the battery topped off, and we received a text at the end of the trip, "Stranded. Car won't start".

We get back, try my key, and the X5 starts right up. Her key.. nothing.

So I'm thinking, "ahh... the key's battery is dead, which means the ignition won't recognize it". Funny thing is though, her key still unlocks/locks the car. Just won't turn over the engine.

Do you think it's as simple as a "dead key battery"? But, maybe the battery can still be recharged? How do I do that? Like.. maybe just put it in the ignition and turn to position 2 and leave it for a few hours??

Or... I've read gnarley stories about having to replace the key for $$$.

Thanks