On my S5 there is an option in the Chromecast app which allows casting. On my Galaxy Tab Pro 8.4, there is no option. I have tested both my S5 and Tab Pro 8.4 with Chromecast. Individual apps, like Youtube and Netflix cast fine, but the Tab Pro won't give me the option to cast the tablet's screen. If you have a Samsung "Screen Mirroring" compatible smart TV it might work (I don't).
On a side note. . . Although I know your client wants wireless, my old Samsung HDMI adapter for my S3 works with both my S5 and Tab Pro 8.4. The Samsung WI-FI All-Share Cast Hub, Wireless HDMI Display Adapter (can't remember the name) might work if you have that plugged into the video source. I didn't purchase it, so I can't test it.