The short answer
Yes — cabbage is a safe, nutritious leafy vegetable.
Why
Green, red or savoy cabbage are all fine and vitamin-rich. Like other brassicas, very large amounts can cause a bit of gas, but normal treat-sized portions are no problem at all.
How to feed it
Wash and offer a small piece of leaf, raw or lightly steamed.
How much & how often
A few times a week.