Outlook 2003 (and above) offers the insert object option.

<insert menu, object, insert object>
   create new, or create from file
   object type: microsoft powerpoint slide, or
                     microsoft powerpoint presentation

The create from file option allows the user to embed or link to the
slide/presentation. The outgoing email format should be html or rtf.

This option assumes that the recipient of the email has MS powerpoint, or an
MS powerpoint viewer installed.
The recipient must also view incoming emails in the sent format. If the
recipient views incoming email in text format (an email security measure)
then they will not see the embedded slide/presentation.

An embedded slide can be viewed directly within the email.
For an embedded presentation, only the first slide is visible. clicking on
the object launches the presentation (like a pps file).

If your customer doesn't have ppt installed, then I am uncertain if he would
even be able to view the slides within his email program (outlook?).
Your only other recourse would be to embed screen captured slide images into
the email.