My opinion.....be honest!
If she got taken, deal with that privately with the dealer. If you've used it and can't return it you may be up chit creek, but if he's a reputable dealer it shouldn't matter. Call em out on it. I'd find the better deal, get it in writing and take that to the place she bought it.
If your GF is gonna get pissed because you're trying to be honest with her and look out for her money and best interest, then you've got another problem. If your relationship works better when you're deceiving each other, you've got a much bigger problem in the future.
I'd handle it first, then bring the solution and results to her. If she's in it for the long haul then she'll appreciate the whole thing.
Either way.....congrats on the birthday gift!