You can try to disable avast when installing the spy software on your computer, another way try some undetectable spyware.
You may mean well, but I think this to be bad advice.
Why have an a/v if one ignores what the program alerts to? No loss to offer this as it is not your system tho, so any dire consequences would only affect that user following that advice, not you.
If the user has imaging software installed, then he/she should be okay when recovered, but, we do not know if that is the case.
You might want to rethink what you posted.