@JaimeS
I’ve been thinking about why raw milk would be better than pasteurised milk since you first posted. Here are some thoughts…….they are a bit random since although I used to do quite a bit with milk proteins in the past…I am very rusty and my brain isn’t what it used to be for recalling facts etc. I have picked my wife’s brain also since she used to work for one of the UK’s biggest milk producers
1) Denaturation of Protein during pasteurisation
Pasteurisation is very tightly controlled to minimise this. Whey proteins are far more susceptible to denaturation than casein so if denaturation is to blame, then whey would be the first place to look. However whey is reportedly easier to digest so any allergic reaction due to denatured whey would be rapid before it’s digested. However, denatured whey also has a habit of becoming “heat shocked” which changes its isoelectric point making it more susceptible to coagulation which prolongs digestion?
It would be good to understand how the symptoms present themselves, e.g. is it a full instant histamine reaction or actually an intolerance reaction like lactose intolerance (bloating etc.). I only ask because of the common misdiagnosis for food allergies generally (9 out of 10 people who state they have a food allergy don’t, instead they have intolerance's due to other reasons).
2) Disruption of the Casein micelle during pasteurisation
Here is a link to the casein micelle
https://www.uoguelph.ca/foodscience/book-page/structure-casein-micelle
Without going into a lot of science, there is a load of these protein micelles that are arranged due to their molecular charge. Heat and other factors can disrupt this. If the arrangement of casein proteins are disrupted (different from denaturation) then this may explain why the body reacts differently to them?
3) Denaturation of Enzymes in raw milk during pasteurisation
Raw milk contains a variety of different enzymes, some are more stable to heat than others:
http://www.raw-milk-facts.com/enzymes_T3.html
However from a food intolerance point of view the absence of lactase in pasteurised milk is the only one that would be relevant. I know you said that your friend was not lactose intolerant but perhaps he\she might want to have a re-look at this? There is an interesting article on the migration of man and how lactase persistence varies across different populations as a result
https://www.sciencedaily.com/releases/2014/03/140313123133.htm
However if they have a full on rapid histamine reaction (wheezing, rash etc.) upon ingestion of small quantities of milk protein then this can be disregarded.
4) Different herds producing different proteins
Different breeds of cows produce different proteins in their milk. It’s possible that pasteurisation has nothing to do with it and it just happens that the farm selling the raw milk has a different herd of cows to the pasteurised milk. The most interesting example of this is A1 vs A2 milk. There are loads of articles on this. Personally I think this has been a little over-hyped in the press but it might be worth considering?
That all I can think of I’m afraid. It might be worth subtly trying to find out whether the allergy is an allergy or an intolerance and whether they have tried other dairy products without a problem (Butter, yoghurts, cheese, goat’s milk etc.). It might help to isolate the problem?