Please do the following to uninstall citrix reciever on you Mac Computer.
Click on the magnifying glass at the top right hand corner
Type in the Spotlight Area Citrix
Highlight the option Citrix Receiver.dmg and Double click to open
Double Click on the option "Uninstall"
Get the client to put their Apple password, Press "Okay"
Once the second box to confirm the Citrix Receiver was successfully uninstalled Press "Quit"
NOTE: If you cant locate the .dmg file re download citrix receiver for Mac and select Uninstall when it pops up should work the same.
Please refer to how to install the latest Citrix Receiver for mac to re install the latest version once uninstalled.