Hi Emanuel,
Here is a good sample to inspire you build your solution:
==============================================================
function MakeAttributesReadOnlyExceptOne(){
Xrm.Page.data.entity.attributes.forEach(
function (attribute, index) {
var attributeName = attribute.getName();
if (attributeName == "FieldLogicalName"){ //The field which you do not need to make read only
Xrm.Page.getControl(attributeName).setDisabled(false);
}
else{
Xrm.Page.getControl(attributeName).setDisabled(true);
}
});
}
==============================================================
Thanks,