It’s the Fel-D1 enzyme. It is possible for a cat to have a genetic absence of Fel-D1, but I am not sure if that is dominant or recessive gene. Siberians are a newer breed, founded in the 1980s by cat lovers in St. Petersburg finding astounding beauty in many of the street cats. They captured the most attractive and healthy and started the breed. This is therefore a natural trait and not one that is bred into the cats. It would be possible to cross breed the Siberian and through selective breeding, possibly get this trait into other cats. An interesting side note, the Sphynx (hairless cat), typically has the Fel-D1 enzyme in its saliva and sebaceous glands, meaning that although hairless, not all that hypoallergenic.
There is a possibility that your friend’s allergy isn’t to the Fel-D1 enzyme but another compound or substance in feline dander. That would seem to be more likely. Personally, I have had a slight cat allergy my whole life. I have owned multiple cats over the years and have had a persistent antihistamine habit. I haven’t had to take one dose since these two joined my life.
Siberians are long haired, triple coat, very muscular, phenomenal jumpers, colorations vary (there are Nevas which are Siberians that have Siamese in their bloodlines - where you end up getting blue eyes and color-point markings in the breed). These guys are full bred traditional Siberians with no Neva back to foundation.