even if they find out she is just his gf they'll still likely turn the tourist visa down. If they were to marry in the US afterwards it would be fraud. It's jacked up and I hate them for making the system take so long but it is what it is.
if they find out a foreign national has a significant other in the USA, the tourist visa is usually denied because they know the odds are good they will get married and circumvent their f*cked up and slow bureaucracy. Besides, he wasn't asking about a tourist visa