The main point of the OP under discussion would seem to be:
"This way, they cheat their professors and get grades for assignments they never wrote."
it seems to me IMHO, that there is no problem with your friend doing any of this. We should first look at this under the lens of common sense before assuming there is any "sin" or technical "Halachic" discussion.
IMHO, there is no reason to think anyone is sinning here; not even 50%. The OP's assumption that anyone is cheating a professor is simply mistaken.
Today's colleges are a vast mixture of institutions that are different from each other as far as ethics and "house rules" are concerned.
I have found, in my experience, that an essay assignment given by a professor to their students, which is supposed to be done on the private "homework time" of the students, to be handed in later, is usually not subject to the obligation of the students to have written it themselves.
This is especially true with the advent of "online" colleges, where sometimes, even so-called "tests" are given with absolutely no supervision to prevent "cheating".
I actually know students who have paid retired professors to sit with them and do their online college work for them together in front of the computer. AND yes, even take tests!
Why?? Because the college is not strict about this.
So let us break down the situation into the possibilities:
1) The student's class was told by the professor that the essay assignment must be their own work and they will be held accountable under the honor system, or face sanctions for cheating, if caught.
2) The student's class was given an essay assignment with nothing said about whose work it needs to be.
3) The student's class was given an essay assignment and the professor told them that they may seek help with the assignment from others.
4) The student's class is actually an online college without supervision.
5) The student's class has an essay which will be written under supervision as a test in a supervised classroom.
Now when your friend is approached by some student to pay him to write their essay, even if it is for a college assignment, we should ask which of the above 5 categories the assignment most likely fits into.
If it is #5, then we do not need to worry, since it is very unusual for a student to show up at a test site under supervision, and smuggle in an essay so he can transcribe from it. It makes no sense for your friend to need to avoid getting paid because he just might be helping such a risk taker. In any case, they will probably be caught, so no harm done. :)
If it is #4, then there is no problem at all. The online college assumes that you at least read the paper you are submitting since you might need to transcribe it to a special access portal online. What's the difference if you created the paper, or you understand the paper?? You have gone through a learning process of some sort, and your overall grade is made of many things; not just that essay.
Secondly, if the online professor is strict and demands it be your own work, then its like case #5 above, because the professor will know the student's level from their interaction online and general participation. If a student who fails a lot of their participation suddenly submits a professional essay... that professor is going to catch on easily. So again we need not assume the customer wants to submit your essay under such conditions of risk taking.
Thirdly, if the online professor never demanded your own work, then the college has set the bar low. It is not your affair if someone buys an essay from you to submit to a college that allows this, is it? No one sinned.
Maybe that's why some companies only hire people from Harvard and Yale as opposed to those who graduated from "BOZO TECH ONLINE" ? :)
If its #3, then there is obviously no problem at all! Yes this happens a lot!
If its #2, then there is no legal assumption or ethical or moral assumption that it must be your own work. So again, no problem.
If its #1, then that is the only case which would seem problematic. However, that is also not a problem.
The student is afraid to submit a purely professional paper that does not fit his own level of academics and personal writing style to a strict professor who knows this student's work. It would be taking too much risk.
Such a student who buys such a paper anyway, will likely read it and then try to write it again in mostly his own words using a lot of the paper as a structure.
So, I believe that anyone who claims this is a sin, has not thought about the practical situation. There really is no issue here for the one selling their work to others for college assignments.
Finally, while plagiarism is a crime, that is only when someone steals the work of another and calls it their own or does not give proper credit. However, in this case, the student bought the full rights to the author's work; so there is no stealing involved. Therefore, there is by definition no plagiarism!
Therefore, the entire premise of the OP's question: "This way, they cheat their professors and get grades for assignments they never wrote. Now, my friend does not know exactly what they do with the works he writes, but we can all be 99% sure that this is what happens."
... is simply a mistaken premise to begin with.
Therefore, any question of Halachah is moot since the reality is much different than the question assumed based on the true facts on the ground.
The only concern is if a student approaches you and says: "Hey, I have an assignment due that the professor said must be my own work or else it is cheating! Can you write an essay for me? Here is some money and some samples of my own previous work so you can even copy my style as you tailor fit your fabricated essay just for me so I can fool my professor! Thanks Buddy!"
You can simply answer: NO.
OR You can say: look, I am not going to help you fool the professor, but I can give you an essay that is easy to understand and very professionally written so you can adapt it to your own style and use it as a base for your own essay? How about that?
If they agree, take the money! :)
A person's hard work and income is a very holy thing. We should never knee-jerk assume that some form of income is "sinful" just because it seems problematic on the surface. We must deeply examine the true circumstances, before declaring a person must quit their living.
That's my humble opinion, based on my Torah knowledge and training.