You're talking about working as a medical practitioner?
Dunno the US, and the recognition of their papers is what you should check. It is possible that an exam or test is mandatory for a general practitioner, however for a specialist there might be more required like doing an internship. Why don't you contact your medical board and ask? I did and got all answers via the web.
I am US trained physican and I am aware of residents who were trained in foregin countries. It would take two years of study, study and study, passing board exams (which are not easy by any means, 50-60% passing rate for foreign graduates), and then at least 3 years of residency. Visit the the site, www.ecfmg.org it has all the information you will need.
True, if the person is not well prepared, she may never get license to practice in US. And yes, getting a residency position is very competitive and you may not get a choice of good city.