As others above have said, if it's for a friend, then do it. If the person is truly a friend (or if you're truly a friend to him/her), then do it for nothing. He/she gets a great model. You're doing it for someone who is not able to do it for her/himself. And best of all, you get to work on a kit that you didn't have to pay for while, at the same time, expanding your area of expertise. This friend of yours is literally handing you the opportunity to practice your hobby.
I like what Don said. Over the course of time, you'll eventually run out of room to display your builds. When that time comes, you'll be chomping at the bit to build something for someone. That's the point I've reached in my life right now. I've built models for my friends and relatives whenever they've asked me to build some for them. I get the sheer pleasure of the build and the pride of having someone else display my work in their house.
I think I actually tend to do a slightly bit better job when I work on a model for others. If it's for myself, then I can accept my own mistakes. If it's a build for someone else, well, I have my pride you know. LOL! If it's for someone else, then I'm most likely to do a an absolute whiz-bang job on it so I don't embarrass myself AND so that they'll continue to ask me to build stuff for them!
Eric