So after several attempts to explain that organically grown mushrooms are grown in sterilized horse manure and therefore NOT Vegan friendly, he gave up and sold them what they asked for.
There are many different types of veganism and many different reasons to go vegan. Not all vegans are vegans due to the ethical implications, many are due to the health benefits and so are still very happy to use leather and manure. Others will use manure as it is a byproduct of the animal living and does not affect the animal to have it collected, and so fits into their ethical viewpoint.
The women asked your husband for mushrooms, not his opinion of their veganism. I think he was quite rude to argue with them about it and to judge them on whether their view of veganism is the right one.
I thought vegans use zero animal products, otherwise they are some form of vegetarians. I'm not arguing, it's just what I always thought. Perhaps I need to research more.
Anyway, from the way the girls acted, they are still rude SS. If they know what they are talking about, it's easy to say "Thanks for the information, but it's fine. I still want the mushrooms."
I'm not saying they were right or not rude but according to the OP, he "tried to explain why organically grown mushrooms were not Vegan friendly but they would not listen. Just got louder and shriller, demanding he give them "organically grown prtabela mushrooms because we are Vegan and we NEED them!!"" and "made several attempts to explain". This to me means that they persisted in asking for what they wanted and he, instead of just giving it to them, argued with them. If I was asking for something and the shop assistant kept telling me why I was wrong and refusing to get them for me, I'd get a bit upset too.
He also seems to have decided that he knows everything about living a vegan lifestyle and what it means to them individually, as he decided that "By the way they acted they had no clue what was involved in actually following a Vegan lifestyle, especially since he noted they were also both carrying purses of a brand that only uses leather to make their products"
The women were pretentious and rude to declare that they needed it because they were vegan, and he was right to give them the knowledge that they were grown in horse manure, but I don't think it is SS to insist on being served what you asked for.
Without getting too deep into it, there are as many types of vegans as there are other types of vegetarians and I have seen vegans having stand up fights about whether honey is vegan or not, let alone manure. Living a life with no impact on any animal and which does not use any animal products can be almost impossible, so a line must be drawn somewhere and it seems to be drawn at many different places for different people (if horse manure is not OK, what about the worm castings that are naturally in the soil, is that also bad, and so on). If you want to know more there are many excellent websites and forums.
(BTW, I'm an omnivore, I just know a lot of vegans so I spent quite a bit of time looking into it to make sure I could cook for them, and then found out how complex it is and so have had to talk specifically with each to find out where their line is drawn)