How Google get to know that the link which is made is made by the person related to that company only?

[Petter]

May be the link is made by his comparator to rank his site down?

[Jake]

That would be "Negative SEO" and is a possibility, in the past Google made allowances for the possibility of sabotage and would simply ignore suspicious links, not they may penalize a site after sending an "unnatural link pattern" warning through Google Webmaster Tools, Google reps have mentioned there is a small chance of ranking being damaged by outsiders.
Their algorithms are pretty clever, they might compare the good to bad link rations, it a site has no good links then bad neighborhood links are more likely to cause penalties.

The recently added Disavow Form allows a webmaster to say, they didn't create the link and don't want it.

http://gadgets.ndtv.com/internet/news/go...seo-281103

http://www.seobook.com/negative-seo

http://www.thewordsmanship.com/2012/07/1...echniques/

http://www.distilled.net/blog/seo/5-easy...ative-seo/