This is just a guess. I would think that the only thing the 4473 form would prove was that she was the original buyer.
For instance, I bought both my pistols new. I might decide down the road to sell it to a friend of a friend. I can't come back with a 4473 and say look, this paper says the gun belongs to me. You get into he said/she said. The 4473 may be evidence, but I think it is far from solid proof.