Try Litecoin mining. (Using GPU).
Suppose you are having good GPUs, which give you a hash rate of 600 KH/s. According to the latest calculation, you can earn 42.21 LTC every month, and 513.88 LTC per year.
Litecoin mining is dominated by ASICs nowadays. Although 100 GPUs is a lot of power, it would probably be more profitable if he chose to mine a CPU or GPU coin instead. For a list, check out:
http://www.cpucoinlist.com/To find the most profitable coin to mine, he could use the calculator on
Coinwarz. Then he could convert them to BTC using an altcoin exchange such as Cryptsy.
try to use your 100 computers in distributed computing (like boinc) in science and make the world better
There are coins which reward you for contributing to distributed computing projects too. For example, you can earn gridcoins by contributing to BOINC projects such as Rosetta@home. Curecoin and Foldingcoin (built on top of Counterparty) work in a similar way except they use Folding@home which is more popular.