You justify working for free, and I get it. Sometimes, I worked for free in my companies, but it built my equity value. I can also somewhat get it if it’s for a good cause—if you personally want a change that your labor effects in the world.
Working for free for someone else without anything of value in return is a different matter. Even if, in the end, you became friends and got a job. Some people in the games industry will say this is normal, but it’s not (and by law in some cases). For example, the UK's living wage is where society decided to draw the line between fair labour relations and abuse.
When it comes to working for free or almost free, I think whether you'd get much better labor conditions elsewhere matters. In many cases, the answer is yes. That's a good litmus test for unfair compensation.