My comment to a person:
Well, you can't convince people if they claim that moral is relative and that it isn't morally wrong to abuse animals. Then you also have to awaken their empathy for animals in some way.:
But most people that claim they are moral relativists do believe that it is wrong to abuse animals.
99,99% agree with the premise that it is morally wrong to inflict unnecessary suffering and death upon another being and then you can easily show that what they believe in requires them to go vegan. A very good and convincing article: http://articles.philly.com/2009-08-14/news/24986151_1_atlanta-falcons-quarterback-vick-illegal-dog-dog-fights