See this discussion: https://moodle.org/mod/forum/discuss.php?d=221190

It is currently not possible to use disabledIf on individual radio buttons inside a group in a moodle form.

The patch provided here uses a combination of the group name (group name+button name) and individual radio button value to trigger the disabled action.
In the moodle form, disabledIf should use the same combination of group+radio button name + button value.
This is only proposed as a provisional workaround. A more elegant solution will have to be found.