Francois Duffar left the company in May and was once its vice-chairman and president. He is now at Cosmos Capital, a private equity-backed business which controls 18.7% of Cossette's shares.
His former partner Claude Lessard, the chairman and chief executive of Cossette, is attempting to repel the takeover.
Shares in Cossette, which are publicly traded, rose almost 60% yesterday after news of the bid broke.
The group owns digital agency Dare, which it acquired in 2007 and Miles Calcraft Briginshaw Duffy and its direct marketing and sales promotion subsidiary Elvis, acquired in 2005.
It bought PR firm Band & Brown in 2004 and branding consultancy Identica in 2003.
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