What is the purpose of the 2nd site?
The age of the domain a ranking factor, but one of many.
If he wants to change the domain your client can tell Google about a change of address in GWT and do a 301 redirect from the old site to the new site.
If the purpose is to have two sites ranking on the first page, a quicker and easier tactic would be to do some barnacle SEO (see http://moz.com/blog/barnacle-seo-whiteboard-friday) and get some of their citation / profile pages ranking on the 1st page for their mold related keywords.
With the recent updates and focus on quality it has become more difficult to try and operate multiple sub-niche sites, and will be much better to focus attention on creating one quality site which is seen as an authority.
