As others have suggested, lactose free milk is the best option. (which is just milk with a small amount of lactase. Your body naturally makes lactase, it's not some kind of unnatural chemical).
However, if your GF has objections to lactase, soy milk is a great alternative. Soy milk foam is firmer than cow milk foam, but it tastes less sweet. You could add sugar to compensate, but I find I actually favor the taste over regular milk.