You need to learn how to re-hair a bow just as you need to learn how to drive a car. Doing either without training (or from a "do-it-yourself" website) invites disaster. With any luck, John's list of tools will put your friend off the DIY idea.

Bow-quality horsehair is easily available from any number of suppliers. It doesn't have to come from Mongolia, nor does it have to come from a stallion; though snobbery and mythology will insist that it does. The only important thing is that the job is done by someone who knows what they are doing.

A bow is not a plectrum. It is a fundamental part of the sound you get from your instrument, and should be taken very seriously. Mine cost almost as much as the fiddle; and it was worth it.