I agree with JCakes - you might be able to salvage them w/ some simple syrup. Are they a bit hard on the top or hard all over? Maybe if you are planning on filling them (and they are harder on the tops) the hard part can be removed w/ coring.
Unfortunately you might have to remake them :(
Best of luck. I did a couple hundred for a party one weekend and they got dropped the night before. Hopefully that doesn't happen this time around (I have to make a few hundred for a party and a fundraiser this weekend as well)
What do you mean by hard?
If it is the icing and that icing is veg shortening based (no egg or butter) scrape it off and check the cake.
If she has used one of the buttercreams based on meringue....total loss as they have visited the danger zone.
Hard lesson learned, but best learned now before the project is someones wedding cake.
Been there, marv and know exactly how you feel.
If they are just baked and not iced maybe you can save them by spraying them with simple syrup, use a spray bottle, it will give you better coverage vs. a brush. Cover them at that point and allow the syrup to work in. It may help to warm the cupcakes prior if there is alot of butter in them. Hope that helps. Sounds like some may have been overbaked to begin with, since you stated only some got hard. If that is the case they will probably need to be redone. Sorry to hear. I think every pastry chef had dealt with something similar.