Ask HN: Competitor is successful with black hat SEO

Hey guys - for some time our startup did organic link building and technical seo. We ranked for a year approximately as our closest competitor. Since the end of 2016 he is increasing the number of incoming links massively by adding bridge sites. These have keyword heavy domains and keyword heavy content with all links pointing to their site. Contrary to seo consultants opinions we've ask ("don't worry") this (for all I know) works for them. They have now left us quite behind in rankings. As we are still not a known b2b marketplace brand and depend on seo this is hurting. Ahrefs/Semrush does not report other large changes in their technical seo or link structure beside this massive buildup of bridge sites. What would you do? We don't want to engage in the same tactics, should we?