It depends, did you formally hire your friend to do work for you? Or did you ask your friend to help you install a fireplace you already bought?
In the first case, if you hire your friend to do his job for you, I would expect to pay his normal rate, if my friend wants to offer me a discount because we're friends that's a bonus. Most companies don't allow the customer to dictate how much they'll pay, friends or otherwise.
In the second case, I would probably take my friend out for dinner or something. Compare it to moving houses, you can hire movers to move your stuff, or you can get friends and family to help for free. Generally you just give them some pizza and beers after. I know my dad has helped friends of his with stuff around their houses that relates to work, he doesn't get paid by them, because he's just helping out in the evening or something.
Conclusion, it depends on what scenario your friends are working for/with you. I know that I've gone to restaurants where I am served by a friend, I generally tip a little bit more then my usual because it's my friend. I've also worked in a restaurant years ago, and if friends of mine came in, I would sometimes be able to get them a discount on their bill. To me that's how friend should be towards each other, generous on both sides of the coin.