I see. I do find it interesting that the customer is unable to eat the eggs if the hens are fed soy. According to
@U_Stormcrow the chickens' stomach is able to completely neutralize gluten, for example, so that the eggs would have no trace of it.
Did the customer try them, and have a reaction? or was the customer simply afraid of even risking it? Obviously, it's not something you would want to push them on...I'm just curious. My family has suffered considerably with allergies, too, so I understand them. I'm allergic to soy, but soybean oil, for example, is generally pure enough for me that I have no issue (allergies are to proteins), and I don't usually have much issue with a limited consumption of soy anyhow. If the customer would risk anaphylaxis, it wouldn't be worth trying--but many allergies are not serious, just troublesome (headache, skin rash, asthma, cough, itchiness, pain, swelling, etc. are all common symptoms, but are survivable).
Certain things like soy, wheat, and milk are hard to avoid these days due to them being in virtually all processed foods. For example, soy sauce typically has _wheat_ in it, and bread nearly always has milk. If having a source of soy-free eggs was desired, I think it would be awfully hard to find, for the reasons you've noted. And eggs are a good source of vitamin B12 and have detoxifying properties.
Well, you don't have to feel bad--you're not obligated to accommodate everyone's peculiar needs. I do, like you, feel bad for them though.