Is it possible for a former SEO to be leaching off our company's website if his site link was placed on ours?

A former disgruntled (we didn't know he was disgruntled until months after we parted ways) SEO provider went to work for our company's previous management (also disgruntled) who of which decided to go into direct competition with our company. Before our company realized the former SEO provider was disgruntled, he asked us if he could place a link to his SEO site onto our company website. We decided this would not be a problem, as when people went to his site, he would also have our website as an example of the work he did. Soon after this, we removed his link from our site. Now we are suspicious that, unethically, he may have linked our site to his and the competitor of ours that he works with, possibly allowing their companies to move up in Google taking advantage of our position.