Both men were friends from 1789 to 1792 but Demolitions was way more moderate than Robespierre. 
In 1792, Demolitions took some distance from Robespierre and moved closer to Danton and Marat. 
Demolitions supported the moderate ideas of these men. 
However, his moderation cost him his life. Robespierre accused Demolitions of being an enemy of the revolution so he was sent to the guillotine on April 5th, 1794. 

Robespierre was finally accused of tyranny and beheaded in July 1794.