Strict Biblical Creationists (as opposed to the general Creationists who
believed that some extra-human force began life on Earth, who are generally
a different breed) will never be happy with any evidence about evolution.
However, in Genesis God clearly gives Noah the animals for food, and in the
New Testament, Timothy suggests that among the traits of those who wish to
deceive Christians by telling them that things like marriage and the eating
of meat, which God has said are good, are not good.

Given this, the adventists' religious belief in vegetarianism strikes me as
very difficult to defend on theological grounds, and I don't understand how
they can continue to maintain that belief.

In general you can make creationists sympathetic by just pointing out to
them that (A) eating meat is natural, and (B) we have evidence that cereal
grains are unhealthy and probably shouldn't be eaten, which there isn't
much biblical contradiction on.

You can also point out that there is much reason to believe that people
changed after leaving the garden, and also after the tower of Babel
incident.  Apparently God changed us, because in the garden we could only
eat the fruit of trees but of course we could never do that now.  Somehow
we got many different races since the days of Adam and Eve too, so there is
much reason to suggest that perhaps people need other foods than they did
in the beginning times.

These are only generalizations, but at least you can probably convince
these folks that perhaps certain foods are worse for them and certain
others are better for them than they thought, and get them to give it a
try; even if the notion of evolution is wrong, that doesn't mean the diet
itself isn't worthwhile.  That's worth pointing out too.