So your mum, along with some others that have commented, are just allergic to the type of fungus (mycoprotein) they use in quorn products. That is like with anything though - some things make people sick and some things don't. Just like some people are allergic to nuts, soy etc. and others aren't. We sometimes don't know until we try the product. Quorn have the mycoprotein written in the allergen advice section of their website and from what I can remember, on their packets too. It really isn't the companies fault that your mother just happened to be allergic to it. It doesn't make the product bad any more than it makes a packet of peanuts bad.
In medication they have penicillin, which is also from mould. Some people are allergic to it and some aren't. I guess it is a similar thing to mycoprotein.
I don't think it is necessary for people to try and ban a product simply because THEY had an adverse reaction to it. This would be like banning products with penicillin or peanuts etc. to which MANY people have had adverse reactions. There is no logic in it.
The only thing bad about the product is they have yet to go completely vegan, but when they do, I will be surely buying the product. I never got sick eating Quorn in the past when I was vegetarian