The surprising effects of the coin made it clear that it did in fact have magical powers, just not in the way you had hoped. Still, the fact that you were able to get your hands on such an amazing artifact was well worth one wish gone wrong. If anything, you could always make the same wish again.
But wait, what would happen to the girl if this wish landed on tails? Would she again gain more clothing? Would any of the physical changes happen? Are you willing to risk having something strange or horrifying happen to this girl you don’t even know? Maybe it was best to just go home and conduct some experiments. To get a feel on just what this magic currency was capable of.
Alternatively, you could make a different wish altogether. Off the top of your head it was fairly simple to come up with various other ways to see this woman naked. For example, you could wish for her to become your girlfriend, or perhaps a prostitute.
But those who play with the devil’s toys will be brought by degrees to wield his sword, and you’re not entirely sure that you wouldn’t fall down a path of misuse and cruelty if you even decided to use the coin. Maybe it was best to just discard it all together and have it be the business of some other wandering soul.