Although it might not exactly fit into this forum I have always found that using the homeopathic supplement known as Pollinosan has been exceptionally helpful for hayfever like allergies.
Otherwise Quercetin is excellent for reducing allergic responses and is made stronger by the use of vitamin C which is also an excellent natural anti-histamine. Also you might want to find out what other foods your friend likes to indulge in, often environmental allergies are made much worse by constantly eating foods a person is sensitive too. This being far worse when they aren't even aware of this!
Plus I believe it's somewhere between 60-80% of all histamine production occurs in the intestinal tract. So suffice to say if they aren't frequently having bowel movements all that histamine just keeps on building up!
The other posters were right in that garlic or onions can help because both contain quercetin (more in onions) as well as being high in sulphur so they'll reduce the mast cell release of histamine before the allergy starts. They're all better at prevention, however they'll help in reducing alleries once they occur as well.