An attempt to gain immigration status this way would be a scam.
No, it is all about money, not emigration.
Nigeria is the absolute mecca for this stuff - it is an industry there...
A bit like the scammers from India who try to tell you they are from Microsoft or your telephone company and for a fee will fix the "problems" with the OS or internet or whatever - I get about five calls a week from these blood-suckers...
I even got one from some guy claiming to represent British Telecom, despite the fact that I am in Australia!
No, this is a scam and money is their object. The girlfriend business is more subtle than that described above because they need to get one's confidence and that takes weeks to months of back and forth communication. Once they think they have your confidence then requests for money for medical treatment or airfares to visit you, or whatever, will be forthcoming. The several thousand dollars involved is usually not a massive amount for someone in a developed country but represent many years of income in a place like Nigeria!
Of course, eventually the victim tumbles to the fact that they have been had - but it is much too late to recover any funds at that stage.
Like I said, in Nigeria this plague has reached industrial proportions...