[I extended my comment some that I wrote in the other thread.. ]
It is sad that they conflict the very basic principle of Jainism. It is objective that consuming animal products is causing Himsa (violence), just like all domestication does. It is impossible to deny that violence is commited against the animals. And according to Jainism human and nonhuman animals are equal, so I don't understand how they can justify enslaving animals and exploiting their reproductive organs without the consent of the cow, let alone the massive killing of 1 million male calves that occurs in India per year because of the milk industry.
Basically their argument is assuming that it is morally right to rely on a long established tradition of eating milk products. They let a long established tradition overrule the very basic principle of Jainism - Ahimsa.
Violence anywhere hurts everyone everywhere. It is impossible to justify violence and at the same time advocate the principle of Ahimsa. Justifying commiting violence against cows for the taste or convenience of milk products, creates such a big 'exception' for Ahimsa, that it justifies any and all violence against any being.
But we all know why the tradition-argument doesn't hold. The lack of understanding is due to the disconnectedness that comes by participating in violence and exclusion towards other sentient beings.
Also this book by Will Tuttle is good in creating the understanding of the necessity of veganism: http://www.worldpeacediet.org/
Maybe it also could help them to understand that the animal slavery has resulted in human slavery, oppression against women, and other forms of oppression that is rampant in our society. This book covers this: http://cup.columbia.edu/book/978-0-231-15188-7/animal-oppression-and-human-violence#.UZAIageHrdk.facebook