Hi there and welcome
Can you restart the Mac and try to install it again.
Tell me if it then allows you to do the installation please.
I Skype, click on Skype at the top left, then choose Sign Out.
This will sign you out of Skype.
The your brought back to the sign in window.
There put her details in and it will then sign in with her account.
Let me know how you go please.