Well, leaning to do it could be one.
Isn't that what hobbies are all about? Also, he could be n the situation that me may be able to e-mail the code to them. This would save having to travel to a specific place to hand out keys.
That depends on how the question is worded. If he said he was doing it to learn as a hobby, then by all means go ahead. However, he made it sound like he was doing it for his job. In that case you have to logically ask whether it makes sense to bother the customers with a custom made machine that may not work when they get there, especially if you could just as easily hand them the keys. I agree about the emailing aspect, that would be cool. Though again, he has to plan to change the code daily and ask himself whether he is secure enough in his job to risk it not working for a customer.
I like the SD card idea though.