I see it as deception, and kind of a mockery of marriage to do that.
There are lots of options for getting that girl citizenship or at least a visa. Student visas are very easy to get, and there are lots of other options toward citizenship.
Besides, I have a friend who married a woman who was not a citizen, and the paperwork and interviews they make you go through is horrendous. The system is very carefully designed to make sure people aren’t doing exactly what you describe. They make sure that you love each other, they monitor your interactions, they check in on you to be sure that you are living together and show all signs of a typical couple. Are all her clothes there? Are overnight toiletries there? If there is any rental or separate housing under your name or her name, it’s suspicious, and if they catch you committing fraud the penalties are severe. They look at how long you’ve known each other, how long you’ve been together, and how long you’ve been engaged. In some cases they even talk to families. Not to mention background checks.
There are better ways to get someone like your hypothetical girl help. Talk to a lawyer, they are always willing to offer help, I am sure! You also have to bear in mind that the very process of exposing her as an person not properly documented to live here, and they think you are getting married to make her a citizen and they deny your applications, she can be deported because you just uncovered her. There is, of course, a very hefty fee associated with all of this. It’s just NOT worth it to defraud the system in this way. There are far safer ways to get citizenship. I am not saying I agree with the system, but it’s too risky and it’s a form of lying and shows badly on the institution of marriage.