The better angle to take here would be to point out that taking an apple from an apple tree does not harm the tree. From the plant's point of view this is actually beneficial. It wants animals to take away its seeds. So this is more alike to taking milk from a cow.
Taking milk from a cow is in no way like picking an apple. Apples are intended to be eaten by any species that wants to, as a kind of quid pro quo for spreading the seeds. Cow milk is for baby cows only. Humans keep cows lactating by forcibly impregnating them, and then taking the resulting calf away. Much suffering is involved, physical and emotional.
