If it's for design only, why not use a div with a defined height and set to scroll? Then you get full seo benefits as well.

You will get no seo benefit by using iframes and google will view it as an empty page, so it is really not suggested. A workaround would depend on why the client requires an iframe specifically. One alternative to an iframe is to to use a programming language to grab the content of the third-party website and dump the content as source code onto your page instead of a an iframe. However, you would then need to further program it to recode any links on that page to go back through the script and process the next page the same way. It would take some coding but it can certainly be done and you would then have full seo benefits.