My first thought in these situations is: how can I turn this into an opportunity? You could of course politely refuse. Or offer a discount on a replacement (a good idea mentioned in another reply). But there's another option... give her exactly what she wants. Exceed her expectations. If you do that, perhaps she'll be blown away and tell her friends what happened. Perhaps she'll be back in the future for more purchases (her girlfriend will need more eventually). Perhaps she'll leave you an absolutely glowing review. Or perhaps she'll do none of the above and your just out the time/effort/money for the replacement.

You have to ask yourself: is it worth the risk to possibly score a devoted, lifetime customer? One who may turn into an avid fan who brings you more business through positive word of mouth? To me, the answer would almost always be yes. It's like winning the long odds bet, and the feeling it brings if it works out makes it all worth it, in my opinion.