If your client missed the 48 hour window that they are given to download a full gallery they can head back to the gallery and click the Download All Images button again. The system will create a new zip file (or set of zip files) and email the link to your client. They will have 48 hours from that point to download from those links.
In the case where it was a purchased download your client can locate their emailed receipt and click the Download link again. They will then get an email with a fresh link (or links) for their gallery download.