My ex friend has a user in netflix (including email and password). Can I somehow kick her out?
Yes, by changing the password
In the settings there's a device log off feature.
And then change password.
Change the password, … Before she changes it and you almost throw out of your account
Still logged-in devices such as cell phones remain as far as I know in the account.
No, it's not like that. When I changed it last time I had to enter the new password on each device.