Complex sentence: The important French watchmaker Abraham-Louis Breguet was a great friend of Arnold, who it seems gave him permission carte blanche to incorporate or develop any of Arnold's inventions and techniques into his own watches.
Split into simpler sentences: The important French watchmaker Abraham-Louis Breguet became a great friend of Arnold. Arnold appears to have given Breguet "carte blanche" to incorporate or develop any of Arnold's inventions and techniques into his own watches.