Since the OP mentioned a client, I assume that this is a book intended for sale. In that case for the Kindle platform hyphenation should be automatic as part of Enhanced Typesetting for anyone reading using a device sold within the last four years or so with recent firmware.
Some e-book formatting can block conversion to KFX format and thus Enhanced Typesetting. The EPUB can be tested using the
Kindle Previewer to determine if that is the case.