Two things stick out to me.
First, SSI_GetValue does not support being passed in a second argument. It seems you want to get the value of this field and check to see if it's true, but the function doesn't know this. Instead we'd want to do this:
SSI_GetValue('CA01_r15') == 1
Second, even with the conditional fixed, the question that remains is when this code runs. If we just add this code to the end of the footer, the behavior is going to be that the browser will run this code just once, when the page loads. But what I think we want is to have this code check everytime the respondent changes their answer. So we'll need to wrap this code with some sort of event so we can get the code running when we need it to. Usually we use SSI_CustomGraphicalRadiobox or SSI_CustomGraphicalCheckbox for this, but we actually do it a little differently when the "Better Lighthouse Library" is included on the page. Please try adding this script to your question and see if it gets you the behavior you want:
<script>
$(document).on('lighthouseRadioButtonChanged', function(event, graphicalObj, inputObj) {
$('input[name="CA01_r15_other"]').prop('disabled', true);
if (SSI_GetValue('CA01_r15') == 1) {
$('input[name="CA01_r15_other"]').prop('disabled', false);
}
});
</script>