If they all point at the same page, Google will just pick one as the "main" name, and ignore the others as duplicates. No real penalty, no real advantage (except for the spelling challenged).
wants to use these to get higher rankings with the SE's
It may have been that way years back, but it doesn't work that way in 2005 - not if the domains all point ot the same content. The search engines will apply whatever rules they want to show only one domain name in the results - and it may not be the one your client would choose.
If there is potential type-in traffic on any of these domain names, you can use a 301 redirect to move that traffic to the principal domain. In any other case, I find it's easier and less fraught with potential troubles just to own the names but leave them unresolved.
Of course, you can develop different content for each domain name you are protecting, perhaps with each one focused on a different aspect of the business. Then, with a modest amount of cross-linking plus a healthy amount of independent link building for each domain, you have other opportunites to play with. But if this development is done just for the search engines and not for the customer, sooner or later you may also have trouble.
