EL expression to make a field read-only based on the checkbox state

Hello Experts,

Is it possible to use an EL expression to make a field read-only based on the state of a checkbox? Specifically, we have two Descriptive Flexfields (DFFs) in the Assignment section: DFF1 (a checkbox) and DFF2 (a textbox). We need DFF2 to become read-only when DFF1 is selected, and editable when DFF1 is not selected.

Currently, we are using this EL expression but it's not working.
#{DFF1.selected == 'Y' ? 'true' : 'false'}

Looking forward to your guidance on this.

Thank you in advance!