If they resemble "hairline" cracks or maybe just a little bit bigger, they are probably the result of too much moisture in the grout when it was mixed. It causes the grout to "shrink" as it cures and cracks appear.
Short of having the old grout cleaned out and replaced with new, (big job) I don't believe it can be fixed.
Structurally, I don't think it hurts much. Visually, they will probably fill with dirt, etc. as time goes on and become less bothersome.
To protect the owner, you may want to consider witholding some percentage of payment 'til he can get back and approve/disapprove.
Dave S.
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