What symptoms does the buyer report the birds are showing? Determining the actual illness in play would factor into where/when the chicks contracted it. As none of your birds are showing signs of illness it makes a very good case for whatever is going on being related to the care they have received under the new owners watch.
Knowing what is going on with the chicks would factor into how I would approach it - if I knew it was not anything related to the care or health of the birds at the time of sale I would explain as much (politely), and if I could identify corrective action they may need to take to help their birds recover I would make those recommendations. If, though, it seemed that there was a chance it was pre-existing at the time of sale I'd take corrective action immediatey in the form of a refund/replacement. Even if you feel it is the buyer's issue it can be beneficial to be a bit generous if you are trying to establish or maintain a good reputation - perhaps splitting the difference and expressing that because none of the rest of the brood are showing any signs of illness the best you can do is a 50% refund or 50% replacement of birds because even when you are in the right badmouthing from an unhappy customer travels quickly.
