Perhaps he cannot build his own mysql2 extension.
If you are using Ubuntu 14.04 or later, give it a try sudo apt-get install ruby-mysql libmysqlclient-dev.
On older versions of Ubuntu, install this: sudo apt-get install libmysql-ruby libmysqlclient-dev
In OS X: brew install mysql
There are also similar questions: 1 , 2 , 3 .
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