If you sign his user out of the device you will lose his presets and captures and also access to whatever plugins he has bought. The accounts are supposed to be personal I think, and as long as he is signed in and you dont know his password you cant access the cloud to find cool new captures.
You can play around with your QC with the current stuff on it, but if you want to add your own 3rd-party IRs, presets, and more plug-ins that it doesn’t already have*, you will need to do a factory reset and sign in with your own Neural DSP account.
*That is to say, you will need to have your own purchased licenses for these plug-ins.
You can also contact the person. If they intended to give you the plugin they did it the wrong way. If its only one its probably the one they chose for free. In that case its not transferable. If it was purchased you or the seller can pay the $25 fee to transfer the license to your ilok account. But you have to be very sure to give them the correct account number because if you send it the wrong person you cant get it back.