Welcome to the forum.
You mention a script to do this, but I don't see the need for a script. But then perhaps I misunderstand.
If the Access Privileges to edit "FEE" are defined by calculation of the type: 3waybilling = "No"
then FMP will verify the "No" status before allowing FEE to be changed. No script needed.
One loophole to consider, being that you're using radio buttons...what if I select a fee of 0.1 and then check 3waybilling back to "Yes". To conquer that happenstance, you might also want to consider a validation on the 3waybilling field that looks at the Fee field to confirm that it is zero or empty. Again, no script needed.
Hope this helps, let us know if it works the way you want it to.