Can my father's smartphone be compromised?
The correct approach in case of a virus/malware infected device (even if it already had been clean-up by e.g. Google) is to totally erase the device (user and system data) by performing a factory reset and re-flashing the complete firmware.

Virus scanner apps are next useless on Android as they usually don't have system access. And even with system access it is still easy to hide from Antivirus software.

The main problem is that the security patches are more than one year old. Therefore a malware app could have exploited the device and installed itself into the system. Therefore there may be still active components in the system that are invisible to you and any antivirus software.

I would also not to hurry up to get the device back in a usable state. People (especially relatives) learn better not to click on everything they see when there are negative consequences.