I'm confused, are you asking about the bacteria Lactobacillus casei or the milk protein casein? L. casei[/] (the bacteria) is found IN milk products (as well as many other places, including your own gut!) and is added to milk curds as part of the cheese making process. Its one of the bacteria that helps make cheddar cheese. It is also the bacteria responsible for most yogurts, is what turns cabbage into saurkraut, is partially responsible for Belgian lambic beers, and basically anything else in food where something is naturally fermented. It can be isolated and cultured in a lab very easily. It doesn't come from a milk protein (that doesn't make sense - its like saying you come from beef...) and it is naturally present pretty much everywhere. SO, if you're wondering if your milk-sensitive dog can eat food etc. with L. casei, I don't see why not - he's probably eating it without you knowing anyway.
Casein, on the other hand, is one of the major protein components of milk. But I have to clarify something: does your dog have a milk allergy or is she lactose intolerant? They're not the same thing. If she is lactose intolerant, things with casein in them might be fine, especially in very small quantities. If she has a milk allergy, I would avoid anything that list casein (or other milk ingredients) as an ingredient. In either case, the particular bacteria L. casei should be fine, for the reasons stated above.