Dharma should be free? Dharma is usually donation-ware.

There is a really valuable dharma book that my friend is in no financial condition to buy (it is $100) but he also does not want to download it for free from me.

Is there any sutta which clearly states that dharma is donation-ware and that it is OK to redistribute dharma content even if it is copywritten?
The Buddha did say:


  "One should not go about making a business out of the Dhamma"


This web page gives a source for the quote. They use a different numbering scheme ("Ud 66") to Access to Insight, which gives it in the verse at the end of "Ud 6.2 PTS: Ud 64":


  One   should not make an effort everywhere,
      should not be another's hireling,
      should not live dependent on another,
      should not go about
          as a trader in the Dhamma.


However, this is on the side of the seller. The seller is putting wealth above sharing the dhamma which is why this normally is considered to give bad karma. This quote says nothing about the buyer. Just because selling the dhamma is not a wholesome action does not mean that taking the dhamma is a wholesome action.

There was no written language at the time of the Buddha so there was no such thing as taking the dhamma which was not given.

You'll have to make your own decision about whether you should pirate the book. The second precept may also be relevant but some consider it to not apply to pirating. "I undertake the precept to refrain from taking that which is not given"