She has an AMERICAN Fiance.  She was tagged as in intended immigrant. She probably disclosed that she is getting married.  They probably asked to whom, she said American. They asked where do you intend to settle, She said in US.  BAM.  At this point you are are intending immigrant, no longer a visitor.  99.9% sure this is what happened.   Intention play a very strong role in US Immigration..  She was not careful and hence could not enter.

hence the need for visa.

Now, people here are blowing it out of proportions.  Before anyone judges me, I am brown.  I am certain that US border official followed procedure.