If the client isn't connecting at N or AC, you won't get higher speeds. If the client hardware is capable, you can try modifying the adapter properties and forcing the preferred band.
Just an FYI - Whenever I see weird issues with Wifi affecting a single client, I usually find that the client is a Windows device that has been using sleep mode. Sleep causes speed issues and other weird issues like connecting not being able to connect to a VPN but no issues connecting to the internet.