Is it wrong to charge a friend for technical help?

As i asked Is it wrong to charge a friend for technical help? Its getting beyond a joke, how do I fix a fuse, my hair straighteners don't work, my urine is blue. It's all the time!! I've typed up an invoice for all the time I have spent this month 37hours @ �25 per hour but he won't pay, I give up my free timeand this is all the thanks I get, Can I sue? whats 40 years of friendship to that mithering git?