Most people who have a friend who is a surgeon have enough sense not to ask their surgeon friend to do them a major operation for free. That doesn't necessarily mean their surgeon friend is unwilling to give free medical advice. A professional photographer is of course different from a surgeon, and he or she may be willing to give free service to friends. But there are some important considerations to keep in mind.
The first thing to keep in mind is that if a photographer is going to shoot an event for you for free, you have to honestly let them shoot according to their own aesthetic sense. If there is some very specific shot that you must have, you can't have it done for free: you must hire the photographer for pay and clearly stipulate the specific shot or shots that you are assigning well before the day of the assignment.
The paragraph above may seem awfully abstract, so here is a hypothetical scenario to clarify it: Let's say there is a brilliant saxophonist named Ivana who is playing a concert at an important Detroit venue, let's say, MOCAD, hypothetically. Ivana asks her friend Alex, who normally photographs business executives for a business newspaper, to photograph her in concert at MOCAD. Alex gets paid well enough for his dull day job, so he doesn't mind doing this for free for Ivana.
The day of the concert, Alex brings his own professional-grade DSLR camera. But Ivana gives him a couple of point-and-shoot cameras and tells him to "feel free to shoot away!" Also, Ivana is wearing this nice new dress and she suggests to Alex that he shoot a medium close-up during her big solo. Alex worries that he is going to look like a rude jackass getting between the musician and her audience in the middle of the concert, and he's also worried that this hasn't been cleared with the MOCAD staff, but Ivana says that she's giving permission for him to shoot and once again tells him to feel free to shoot away.
The concert starts and Alex is still under the impression that he is free to photograph the concert as he wants because he's doing this for her for free, and the medium close-up during Ivana's big solo was just a suggestion, a suggestion that he ignores. After the concert, Ivana is upset, as if Alex had killed her dog. Ivana also asserts that she was going to pay Alex $100 for that specific shot, but that's the first time Alex hears of this.
That was a hypothetical scenario, one that you can play through again with slightly different circumstances. Play through it again, but this time, right off the bat, Ivana very clearly tells Alex that she has bought herself a very nice new dress she'll wear for the concert, and that he must get a medium close-up of her during her big solo at concert. Alex says that this kind of feels like work at his day job a little bit, but he's willing to do this for her for a small, symbolic fee. Ivana offers $100. Alex says $10, and stipulates that Ivana may use the photo on her personal website as well as her SoundCloud or ReverbNation page, but any other use must be cleared with him. Ivana accepts.
But Alex will not take the money just yet. He talks to the MOCAD staff, who are concerned that people who pay a cover charge to get in the gallery and attend the concert will be annoyed by a photographer blocking their view of the lead musician during her big solo. (In real life MOCAD has charged a cover charge for some concerts, so this part of the hypothetical scenario has a basis in reality).
So, as a compromise, the MOCAD staff sets up the stage well before doors open to the public. Ivana gets on stage, runs through just enough of her solo for Alex to get the shots she wants. The MOCAD staff opens the doors to the public, and people sit down for the concert. As the concert goes on, Alex takes more photos, but at no time does he get between the musicians and the audience.
The work of a photographer is not the same as the work of a medical doctor, and it could be argued that the photographer's work is not as crucial. But a photographer still deserves some basic considerations when asked to work for free.