UK address databases are nearly always based upon the one that's made available to commercial organisations by Royal Mail. If an address isn't on that database you can get Royal Mail to add it by using their enquiry form:
http://www.royalmail.com/personal/help-and-support/I-cant-find-my-new-build-house-listed
However (in order to save money) many companies and organisations don't always pay to get the latest version of the Royal Mail database; they work from an older version for (perhaps) a year or two, or for even longer. If that's what's happening with the online firms that your friend is trying to order from, there's nothing that she can do (if her address is already on the Royal Mail database); she'll have to wait until those firms buy the latest edition from Royal Mail.