I think she's just getting one
betta, Sir. If she gets a cup baby or orders from a breeder, he would have had his own water for a while (shared with siblings when he was a teeny baby though).
If the very diluted little bit of cup water left in the bag had any nasties in it, I think he'd most likely have the problem himself. Or would the store possibly use water from other tanks? If so, I'd worry. If not, which is riskier: having a tiny bit of cup water in the tank or risk tearing his fins and stressing him by netting? I'm not really disagreeing with what you said, just asking. Personally, I'd avoid the net but I'll be interested in seeing what you and others say.
If she gets one who's been in a tank with any other fish or even a snail, no doubt she should net him.