Funny, i run into this all the time. Friends come by for a detail, you feel funny charging them, but remember business is business.
You do not have to charge them full price but your time and services are worth something.
If you friend was willing to help with the detail and he brings some BBQ and you make a cool day out of it, then i would not charge as it would be like hanging out. If he drops it off and picks it up later in the day I would charge him.
Friends and Family can be the worst customers!!!