Something to consider...
If your customer has Office 365 Enterprise E3 or higher then DLP is already built in and available for Exchange and Outlook (more apps will be DLP-aware in the next release).
If you're planning to use the DLP feature built into O365 or Exchange 2013+, I will point out that the "out of box" DLP Policies which are included with Exchange are very basic in most cases. As an example, the HIPAA policy does not detect all elements required for HIPAA compliance.
If you go this route, I suggest looking at the DLP Polices from Nucleuz which can be dropped into Exchange for a near turn-key solution. The benefit with this combination boils down to cost savings since the DLP infrastructure & management is already available from Exchange, and you only need to obtain premium DLP Policies.
Hope that helps.