There are a lot of factors to consider here. Let's assume your friend will cover materials cost.
How much is your time worth to you? How much do charge/earn per hour in other endeavors, and how much are you willing to sell your time for? Consider yourself as charging by the the hour--depending on the method and speed, hours can add up quickly.
Do you care whether you make a profit from this order? You may have different motivations for taking an order--to make money, for the joy of rehairing/something different to do, practice. Remember, if you use a slow rehairing method/haven't invested in a rehairing tool, a rehairing job could take a lot of hours. Let's say you charge $35 (some people charge less than that for a complete custom) but it takes you five hours, you've only made $7/hour. Are you okay with that? Now let's say you charge $20 for a 5 hour job. The outcome of that math is even less lucrative!
If you enjoy the task and it's relaxing to you, maybe it doesn't matter if you make even less than that. But if you would rather be doing something else, maybe you would prefer not to sell your time at less than you could be earning doing something else.
Would you like to try this out just to explore it? Maybe you could buy a rehairing tool, and future rehairing jobs would go a bit faster. Then maybe you could rehair something in an hour--suddenly $20 for the job is not doing so badly! And even if you decide you don't want to do it again, now you've got a fancy new rehairing tool out of it.
Check on the tutorials thread--I think it also has rehairing services. See how much people are charging. Also, if your friend doesn't know a lot about the seedy underworld of ponies-for-pay, maybe you can gouge him or her! What could be more gratifying than taking money from a sucker!?
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