score:3
I think it unlikely, but it is certainly possible. The best thing she could do is to be able to prove that you do not intend to stay in the US. She shouldn't bring this out, however, unless things start looking bad.
As with any visitor, she should be prepared to show her own ties to her home country. She should also answer truthfully if asked about her plans for accommodation, by saying that she's planning a road trip with you and that you'll be deciding where to sleep as you drive around. There is no strict requirement to have every night planned at the time of arrival.
I think there's a good chance that she'll be admitted without going to secondary inspection, but mentioning getting married could cause her to be sent there. If that happens, you may want to be available to receive phone calls if CBP decides to see whether you can corroborate her story.