How to scan and detect spying software

Maybe, but it doesn't matter.

There's no matter about "Legit" tools getting a free pass-- The simple point is that Anti-malware and Anti-virus tools can only reliably check for the tools / spyware / viruses that they have been programmed to check for.

Even if legit monitoring tools are not given a free pass by the detection tools, you still have to face the issue that there are countless spy programs which are not detected, simply because they are not known about. There will always be software which will slip through antispyware tools.

If you suspect that there is monitoring software installed on your system, the only 100% guarantee to clear it off is to format your system and reinstall. Antispyware tools can only tell you what is not installed-- they cannot tell you what is installed.

To quote one of my other answers here:

The first rule about computer security:

If I can touch it, I own it.

There are no exceptions to this rule. Physical access = game over.

Besides the concerns about your friend's laptop, you should tell him to change all his passwords for email accounts, logins to websites, home banking credentials, and so on, because they could have been read and stolen from the landlord.

Of course, if you suspect that his computer was put some malware on, he should perform these tasks from another machine.