Working for free means you are not a professional ( by definition ). Never is is more costly and time consuming to use a non professional to do a professional’s job. Would you want a 1st year student to do your heart bypass? We put our trust in professionals to get the job done right and the first time.  I’ve aleays found clients can always find the time to do a reshoot when someone else messes up but fairly the time to do it right first time. If you choose to do something for no $ make sure you get something for it like an ad in their program, some business cards printed or a link to your web page. I have donated my services to those I feel I wan to but I never donate my assistant’s fees. That’s a hard expense.