Beware of those services. It is very easy to apply for a visa, there is no need to waste money on it.
What is the intention of the person seeking a visa? If she wants to visit as a tourist, she needs to fill out a tourist visa (B-2) application which consists only of one page and then get an appointment at the American consulate for an interview. She will have to show good reasons to travel and, more important, enough funds and firm ties binding her to her home land, so that it will appear reasonable that she will leave the States again.
If there are issues, why the person would be denied a visa, she would have to consult with an immigration attorney. These "services" cannot do anything. Most of them charge a lot of money just to fill out the easy application form.
Of course, if you are seriously contemplating to marry her, and you are an American citizen, she should contemplate applying for a fiancee visa. There is some more paperwork involved in this.