My boyfriend's Google account was added onto my phone. How do I delete it from my phone and if I do, will it also be deleted on his phone?
You can only remove the account from your android device.
For that: Open the Settings menu on your device. Under setting choose Google.
You can find the google account added in your device. choose the one that you want to remove and there will be a button that says "remove account" click on that.
That's it you can not see more that Google account.