[NTG-context] MacOS+TeXlive: installation problems

[Tomas Hala]

Hi all,

one student of mine asked me for a help with installation TeXlive 
(from http://www.tug.org/mactex/mactex-download.html) 
on her MacBook Air 16 with macOS Catalina.

Despite the fact that the site states 
"All install packages are developer-signed and notarized by Apple.",
she got:
"Cannot be opened because its from unidentified developer, MacOS cannot verify that this app is free from malware."

What could it be? 
I have got no experience with MacOS, so I would like to ask for some hints how we can fix it.

Thanks in advance.

Best wishes,

Tomáš