Assuming that your employee has tried to install the official CCleaner (and not a dodgy repack from somewhere on the web) then I suspect that what you are seeing is normal behaviour when installing CCleaner an a home machine.
Support should be able to confirm that for you.
However it is probably not behaviour that would be expected, (or allowed), in a business network situation.
(There are business versions of CCleaner designed specifically for IT departments to deploy over their networks).
Presumably your employers are prohibited from installing their own apps on your business machines.
CCleaner home version does require elevated permissions. (Hence the 'skipUAC' option, which although created during install is normally switched off by default).
It does scan to see what CPU is running and it does scan for the Windows version in use, so that it knows what/where can safely be cleaned.
It does scan for other user accounts, an Admin user using CCleaner Pro can clear junk files from other user accounts.
It does scan for installed applications and drivers, it does that in order to know what the user may want to update.
It does scan for startup apps, and other scheduled tasks, so that it can put uneeded ones to 'sleep' if the user chooses.
It does contact various servers to get up-to-date information, such as new application versions or new driver versions.
etc, etc.
In short many of the actions that CCleaner does on install, and during use, are not exceptional for an application of the type.
Most home use AV/AM applications recognise CCleaner, they know what it is, what actions it takes, and that it's actions are not malware.
But if your business cyber security is not expecting CCleaner to be installed on a network endpoint, which it obviously wasn't, then those actions can look typical of what a malware might also do.
In those circumstances then most of those actions are are going to trigger your protection application's alarms if a person tries to install a CCleaner home version on your business networked environment.
Again, there is always the slight possibility that your employee has acquired an installerr for somewhere on the web that could have been tampered with and repacked.
EDIT- I see that your employee says that it was a download from the official site.
In which case malware can probably be ruled out and I'd suggest that it's almost certainly simply that your buisness cyber security is not recognising CCleaner home.
A case of 'user error', - trying to install something that they shouldn't have on a works computer.