If she has immigrant intent for the purposes of the visit, she can be denied entry and sent back even if she has a visitor visa.
I strongly recommend an appointment with a competent and experienced immigration attorney before you complicate the situation. Good luck.
This is general information, not legal advice, and does not create an attorney client relationship.
Marriage is permissible. You may have some issues with US immigration (CIS) if you file for resident status in the US. As a tourist your fiance must have the intent to remain in the US temporarily. If you/she file for permanent resident status in the US CIS may construe this to be pre-conceived intent to essentially use the non immigrant visitor status when her intent was to marry and remain in the US. You should discuss with an immigration attorney before preceding.
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Hello Sir. Very good question. I handle many such cases. One thing I need to be careful of is visa fraud. Specifically, I dont want the USCIS to deny her adjustment of status by alleging that she used a visitor visa to come to the U.S. though she had a preconceived intent to remain here, and not simply to visit. Marriage to a US citizen does not resolve the above issue. This case must be handled properly. Also, I need to look out for any other issues. Best regards.