licenses vary, according to the creators wishes, so you really should check out the font license your customer wishes to use.  There should be an end-user license attachment to every font you download or buy, or contact the vendor.

As a designer, both you and your client may need a license to use the font legally, again, check the EULA to see if it allows you to sub-license a font.  In some instances, you may even need a license for every computer the font is installed on.