"That friend a great man's ruin strongly checks, who rails into his belief all his defects"

This quote by John Webster is describing the idea that a true good friend is somebody who can help avoid a terrific man from destroying himself. A true good friend will not simply accept all of the great guy's defects and mistakes, but will instead point them out in an useful way. This is done out of love and regard for the terrific male, as the buddy wishes to help him become the best variation of himself. The friend is not trying to be important or judgmental, however rather is trying to assist the excellent guy recognize and resolve his own shortcomings. This quote is a suggestion that true friendship is not about blindly accepting someone's defects, however rather about assisting them become the very best version of themselves.