a friend of mine wants to marry an immigrant so she can go and get her kids so they can become U.S citizens as well.
After 3 years of legal permanent resident status, spouses of US citizens can apply for their citizenship. For everyone else, it is 5 years. However, that is a very simplistic answer as there are other factors to take into account. It takes about 6-12 months US citizens to bring an immigrant spouse to the US and get Legal Permanent Residence Status (or to adjust status in the US of a spouse that is currently in the US). Once the card is issued, you can apply for your citizenship in three years. Then, it takes about another year to get your Naturalization paperwork approved, pass your test, and be sworn in (if you meet all of the requirements). So, in reality, processing times and waiting adds about 2 years to the total amount of time that it takes to get your citizenship. If you are trying to fin out how he can bring a potential spouse and her children to the US to live and possibly work, then he has other options like a K-1 fiance visa or a IR1/CR1 visa.
The challenge with simplistic answers is that the U.S.C.I.S. works on a different time frame. Each applicant may have different issues to overcome. If the foreigner, now potential immigrant, wants to immigrate, then it may depend upon whether they lawfully entered the U.S. Otherwise, a series of complications can arise. Assuming that they made a lawful entry in the U.S. or were eligible to process in the U.S., but had all needed documents, and were not ineligible for some reason, then the processing times may be shorter.
The question's writer does not indicate whether the potential spouse is in the U.S., which can have an impact on processing times, as well. Unfortunately, if the writer is a lawful permanent resident, then it may take years before the foreign spouse can process due to limited family visas in the F2A category.
In these situations, understanding how the system works is best undertaken before marriage in some situations. If the friend merely wants to help someone out, and the intending immigrant marries solely for the purposes of immigration, then they can be permanently barred from lawful immigration based upon what is considered visa marriage fraud.
The above is general information not meant to be legal advice. I strongly recommend that you schedule an appointment with an experienced immigration attorney. You should discuss all of your questions, so that you appreciate the situation and how to proceed with the situation.
A spouse of a U.S. citizen can obtain lawful permanent residence within a few months. Each USCIS District Office has different processing times. You can view these processing times online
Regarding U.S. citizenship, once the spouse obtains a "green card", he or she is eligible to apply for naturalization after three years.