Your hubby can be lucky with perhaps just a change of tie if need be, but you will need two outfits i'm afraid! Maybe you could hire a ballgown? Look inthe phonebook under costume hire. Or if you know somebody who dressmakes, go to somewhere like here and buy yards of satin and have a ballgown made cheaply
http://www.cheapfabrics.co.uk/thestore/product _info.php/pName/crepeback-satin-4
Have a good browse at that site, they are very helpful if you phone them up too, they'll send you swatches of fabric
Or try and utilise what you've got in your wardrobe for the daytime, perhaps just buy a new hat?
If either of these ideas fail you, then you can always pretend that you can either go to the afternoon only or the evening venue only , invent a holiday or meeting somebody from the airport etc,. Good luck