I probably have a different allergy than your friend (mine's more of an intolerance than an actual allergy), but I also cannot eat cow's milk cheese (or any cow's milk fat for that matter), or goat's milk cheese. Sheep's milk is also fine for me.
However, in most countries outside the US, mozzarella is not made from cow's milk. It's made from buffalo milk. For me, even though I can't have anything with cow's milk fat, I have absolutely no problem with buffalo mozzarella. It's pretty easy to find buffalo mozzarella, and although it's more expensive than cow's milk mozzarella, it's worth it to have pizza!
I too am looking for a good melting sheep's milk cheese (or I guess buffalo milk would be fine too), more like gruyere, so if you have heard anything elsewhere, please let me know.