One of our staff accidentally deleted a folder of important financial records which was only discovered 7 months later when doing an audit, how do we recover these files and folders?

Before moving our company into Office 365, understanding file recovery is important, so please consider the below question.  Maybe Office 365 can supply this solution, or maybe there are third party products that we can use?

"One of our staff accidentally deleted a folder of important financial records which was only realised 7 months later when doing an audit, how do we recover these files and folders?"