Tis an interesting one, legally...
They provided the coupon, he used it. If the Terms and Conditions do not explicitly state terms or conditions of use, then the law gets rather black and white quite quickly. In the same vein, a contract that is unusual by normal standards is still a legally binding contract, regardless of norms (provided both parties understand and there was no deception).
SS90, you refer to Offer and Acceptance, the basics of forming a contract (which is what purchasing a can of baked beans in a supermarket is). The shop offers you a product at x price (displayed on the shelf) and you accept by taking it and buying it. Changing the price yourself and trying to trick the store is fraud, you knew the price, and set about changing it in an unauthorised way. Now, if the store provides a coupon to discount the product, and you use their coupon, there is no fraud. There are indeed examples of coupons (or specifically, coupon codes) being re-used, the first one I think of is online pizza ordering often uses it (but agreed, not in the exact same purchase). If he had changed their system, to accept his code multiple times, then there is definitely intent to defraud, but he simply used what was provided by the store (albeit in an unintended fashion).
Using fake money knowingly is completely different. You know its fake, its absolutely fraud.
Don't trust my word tho (see, a disclaimer!)

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