This is a really good question and can usually be answered in one of two
ways. First, run a search to see for which SERP they are ranking #1.
Typically, if a client dismisses the need for SEO based on a current top
rank for a term, that term is mostly like what we call a “vanity” or “trophy”
phrase that looks really good on paper. Often, however, this trophy phrase
does not convert traffic at the rate of less popular but more focused
phrases.
In addition, no matter who the client is, the site doesn’t rank at the top for
everything. Highlighting core terms for which the prospective client’s
positioning is less than stellar and having that prospective client see its
competitors atop the list will bolster your argument for implementing a
comprehensive search program.
Many times, there will still be significant opportunity either in the form of
secondary results or long tail. For example, if they are the ranking number
for “snow globe,” you can use alternative domains to help them rank
positions 2 through 10. If that is not an option, you can widen their reach by
going after long tail terms by building pages, content, and links for those
terms. If they are ranking for “snow globe” use their relevancy and current
link profile to target terms like “holiday snow globe,” “winter snow globe,”
and so on.
If this is not the case (say, if Wikipedia is the potential client) the answer
may be that they really don’t need your help. It is much better to under-
promise and over-deliver than to struggle with a client who you can’t help.