How long after conception does a woman become pregnant.?

I have a friend who visit his so called on again off again g/f back in Mexico in Septemter between the 13th and 21st, she lives in Mexico. Just a couple of days ago she stated that she is 4 weeks pregnant, she has been pressuring him in to marrying him since she came out to visit him for the very first time in March of this year. I believe she is just using him to get her papers to become a US citizen. He has tried to break it off with her a few times but she is very persisitant and keeps texing him and calling him. The last time she called was to say that she was pregnant and that was a week after they broke up.