If I understood your post correctly, Itza, your niece printed material that she had worked on with her friend.
So the answer would be "Yes, I printed material for the project that we did together. This is different."
I'm not sure that charging people for printing is a good idea, but something like "Oh, Friend, you want to print something? Well, you can drop by at [unconvenient time]. Don't forget to bring paper!"
(That's what I do. Be helpful... but not too much!)
If the friend starts telling others that they can use Niece's printer, I think that would be a good occasion to practice "Why would I want to do that?" (The only valid answer, of course, is "Because I will compensate you.")
Or just plain : "Friend, it's my printer, not yours. Don't tell others that they can use it."