It will most likely hinge on whether or not the gun shop personnel can reasonably claim that Spengler was merely there to assist due to, say, Nguyen's inexperience with guns. This becomes very plausible, since Nguyen is a woman.
(Before you go after me for being sexist, consider out of the women you know, the percentage who are knowledgeable about guns. If that number is as high as 1 in 3, I would be surprised. Hell, I don't think that 1 in 3 of the MEN that I know are knowledgeable about guns.)
Also, bear in mind that unless the clerk knew Spengler personally, he had no way of knowing that Spengler couldn't own a gun, so it wouldn't have necessarily rung alarm bells. You also have the situation that they were neighbors, so there is a valid personal connection that could explain his presence. His age compared to hers would also support the idea that he was merely there to help her be informed. Not buying ammunition doesn't mean much either. I rarely buy ammo at the gun shops - they are usually way more expensive than going to Walmart, or (in my case) Ventura Munitions, a local store.
Having said all that, I don't think they can prove that Gander Mountain personnel should have known it was a straw purchase. I have gone along with others when they bought weapons, in order to help them with their decision, and it wasn't a straw purchase. Also, the Don't Lie program is NOT an industry standard, it is a suggestion. That means it has no force in law or regulation, so not using it doesn't automatically cause liability to devolve onto the Gun shop.
Based on the facts in evidence, I don't know if I would have made the sale. It would have depended a lot on what was said, who said it, and how it was said. In other words, the type of evidence that isn't available to the jury unless there is a recording of the purchase.
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