[@Belle] Hm, I don't know how the fruit thing would work, but...yeah, that might do it.  If the cure is simple, though, there's no reason to keep Lyra in Spring.

Unfortunately, because the fey know little of science (which makes an excellent reason to use it against them!) it also means they would have no way of employing it themselves.

Unless.  Perhaps an exiled fey (such as the one I had the vague notion of before) with ulterior motives offers to let the Winter Queen use something he's cooked up?  He sees it as a test, she gets a way to hurt Feoras, and Lyra needs extensive care.  Exiled from Summer, most likely, though I could see him being from another court.  He'd be in contact with either unaligned fey or low level court fey, but not allowed in the fey realm himself.