Cooking

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.

Try oat milk… it foams much better that the other alternative milks. There are some brands of oat milk that have a thicker option that I enjoy.

How about lactose-free milk, i.e. where they have added lactase to neutralize the lactose? I never drink milk myself, but if I ever need milk for a recipe, I buy the lactose-free one.

lol..skin milk

FYI, lactase is a complete solution for some, and does absolutely nothing to help others. That's very likely the "unknown reason".

Soy and rice milk both make a perfectly passable latte, for those blighted with intolerance for the to real thing. Like most milk alternates, it's fantastic as soon as one has forgotten how good actual milk was.

As others have said, lactose free milk! As another option, my local vegan cafe also offers things like oat milk and coconut milk - and while they don't quite give the same foam as regular milk, I reckon they make a much better tasting latte.

Skim milk still contains virtually all of the lactose, by the way. Lactose is water soluble, so it remains in the milk when the fat is skimmed off.

If you're looking to get a similar texture to whole milk, your best bet is probably an alternative milk with a similar fat profile to whole milk. That's what makes whole milk the ideal milk steam so nicely. Many alternative milk brands will have a "barista" version of their milk that is intended to steam as nicely as whole milk. In my own experience, these have had much better texture than lactose-free whole milks, but you might have to hunt for a brand with a flavor that your gf likes.

What about lactaid milk? I think if you use a whole milk version of lactaid it should foam like regular whole milk.

Latte means milk

Hows this gonna work?

Anyways

Get regular milk.. but in a lactose free variant. This exists. At least where i live.

Lactose free milk foams up fine in my experience. You can also get 'barrista' style alternative milks that are made to be able to foam up like cows milk. Usually these are in baking or health food isles.