How can I avoid the requirement of using friends or acquaintances to complete coursework?
I agree that this concrete instance is somewhat problematic, but as usual it's a sliding scale. In practice, there are certain learning objectives where you as a student can't help but interact with the outer world, which usually requires them to spend some amount of time or work on your course work.

To give another real-world example, for a teacher's education course on diversity I am currently doing I was recently asked to interview an international student w.r.t. their experiences at our university. This required me to find an international student who was willing to commit 30 - 60 minutes of their time to talk to me about diversity. I found this assignment quite reasonable, as it's clear to me that the general learning objective underlying this exercise can't really be achieved differently.

For me, the difference between your example (which I agree is problematic) and mine (which I think is not) is that in my example, the student did not actually have to do any "work" alone and by themselves. They did not write or submit anything, I actually had to do all the work related to writing up the analysis and report. The student "only" had to commit the time, which is not nothing, but the barrier is certainly lower than if the student had to, say, write a report themself.


  Assuming I was the student in question and asked to find reviewers for my game demo, what is the best way to refuse?


I would ask the teacher whether you can also do the assignment in the way that you as the student make a few review videos with your friends. Basically you would still do the assignment, but you would take over all the "work" of making, editing, and uploading the videos. Your friends would only need to commit the time to look at your game and speak their thoughts into your camera. This would sound like a sensible compromise to me, while accomplishing the same learning objectives for you as a student.