He was trying to go for the "do something ridiculous and stupid that she will find charming" or something like, to be spontaneous. I don't know the full story, but it seems like bad execution on his part. A similar tactic can definitely be very effective if you do it right.
For example, he shouldn't try to "ground" his request for a lunch date for a key by saying that the key is for a bike. She could easily refuse the entire request by saying that she doesn't want the bike, which is what happened in this case. If instead he said the key was for an alien spaceship or the key to save the world or even something like a private jet or yacht, it would've been better. It's obviously ridiculous, but it would make the offer hard or impossible to refuse, even if she had a boyfriend.