I'm pretty sure what you're trying to do is not allowed under Section 1.3 of the AWS Promo Credit agreement. Personally, I would be wary accepting "free" credit that I may end up being on the hook for. https://aws.amazon.com/awscredits/
IANAL but I don't see that a violation. As long as he isn't selling or renting it out for a price then he is fine. I have a feeling that provision is there to make sure people don't try to sell credits, screw over the buyer, then the buyer tries to complain to Amazon. Steam has a similar stipulation - and a lot of people are pissed about it. Basically you can't buy or sell steam accounts - and if you are caught Valve…
Seems pretty clear to me...