Actually, let's use wedding photographer just as an example. Shooting weddings is your lifelihood. If you don't charge, who brings home the "bacon".
How many freebies can you give away. With wedding photographers, there are only so many good days (weekends etc) in a month, a year, where do you draw the line from free loaders?
Maybe it's my up bringing, and my relationship with my parents. What they will do in a similar situation, is to ask the friend/relative to call me directly. They will warm me ahead of time about the call, and I will politely decline to shoot the wedding. Give some excuse. I will then recommend someone to shoot the wedding, and it's up to them to go with my recommendation or not.
Another really overated thing, is loosing face. Most of the time, the party loosing face, got themself into that situation, cannot get out of it, and demand others to bail them out!