The best statement that always keeps me on track is:
"Let's pretend for just a moment that we are in business to make money"

I can't tell you how many times it helped me laugh a little and re-focus.

When I was in the same predicament (sp?) - I just asked my friend how much of her product she was willing to give away for free to help someone. The quizzical look in response gave me the opening to say "this is my type of business and I need to charge for my time the same that you charge for your products". We had a good chuckle and have remained friends.

I hope all works out for you.