Start with optimizing the page he wants to rank (the homepage) for his primary keyword, this is really basic SEO! Google ranks pages based on relevance, make the target page relevant.
Next, optimize the entire website by using keyword-rich anchortext on all the internal site links on every page that points at the target page.
Finally, get some quality backlinks from high PR pages on external websites that use the primary keyword within the anchortext.
These are all signals to tell Google which keywords are relevant for each of your individual pages. It’s important to remember, search engines rank individual pages, not websites.
I thought you were saying the homepage doesn’t have the keyword in it. After re-reading your question I see that you were referring to the inurl: property.
Rather than lowering the total relevancy signals of the interior page, I believe you will better serve your client’s needs by increasing the relevancy of the homepage. There are a number of key signals of relevancy that Google provides search operators for that will help you narrow down the areas that can be improved. Obviously, the sum of those key signals is greater for the interior page.
Use the following keyword filtering search operators, in combination with the site: operator, to see how your client’s homepage compares to the higher ranking interior page.
We already know that allinurl: is nonexistent for the homepage, however, it is just one of the primary signals and you need to see the homepage outranking the interior page for the other primary signal groups. Any of these signal groups that are ranking your homepage lower than the interior page indicate where your optimization opportunities lie.
Once you have optimized all of these (except the inurl: ) primary on-site signals, you then should use the allinanchor: keyword search operator filter, without the site: operator filter, to see how your off-page optimization for those two pages compare. It could be that the homepage recently lost some powerful backlink(s) or that the interior page has gained powerful backlink(s) from external websites.