I'm pretty sure - without being a lawyer - that, if you let someone have your credit card that would be seen as implying consent. I doubt if there's anything illegal in using it under those circumstances.

I would think this would apply, even if the credit card comopany had rules against it - those rules wouldn't have the force of law, and any penalty (such as cancelling the card) would fall on the cardholder rather than the person to whom they lent it.