Disclaimer: I am not a lawyer nor have I ever played one on the internets.
Laws vary and you'll have to check with your state.
1. What you're doing is perfectly legal. Go to the FFL and claim the gun YOU bought. (YOU might HAVE to be the one to do this cause you are the one that actually bought the gun.)
2. Wrap it up and give it to your wife.
3. When she takes possesion of it Christmas day call the cops turn her in for a felon in possession and live happily ever after!
Kidding. Gift her the gun and then see if she has to register it in your area. IMhO after reading scores of these questions you're okay.
PS: My wife wanted to get a Les Bauer 1911 .45 for me for Christmas but the best offer she got was a Hi-Point 9mm, and she still might take it.