I suspect she was using full auto before... but it knows it's in manual now... I'm wondering if it hasn't fully registered something.
I am not sure if the D3300 has the same set up as the D3500. But I have just put my D3300 in full auto mode, and the ISO option is greyed out, and I am unable to select ISO value option via the menu panel. I can't even select ISO option with the fn button and rear wheel dial, even though the fn button is assigned to ISO. If I also try and select ISO using the rear info button, the ISO can't be chosen. So I could not even mess about with the ISO option, thus leaving it not selectable when going into other modes such as full manual mode, like you are experiencing now.
I have tried to replicate various combinations, such as auto ISO to ON / OFF with highest and lower values, and going back and forth into aperture / shutter / full manual modes, and I am able to select ISO value in modes.
NOTE *
When Auto ISO sensitivity control is ON and you set the ISO to your maximum, if you then go and turn OFF the sensitivity control, the maximum ISO value that you chose is greyed out, and you are not able to select it, to change the value.
Check to see if the sensitivity control is set to OFF. Try turning the Sensitivity control to ON.
I get the message
This option is option is not available at current settings or in the camera's current state. The message goes away, when I turn the sensitivity option to ON.
Or at least that how it works on my D3300, not sure if the D3500 is the same.