Non Dependent Deductions and PIP

Hi I've recently been awarded the daily living and mobility elements of PIP and am in receipt of the housing element of Universal Credit. My son lives with me most of the time and he works but I read that if you get the daily living element of PIP that Non Dependent Deductions shouldn't apply. I enquired with the DWP via my journal but they said it only applies if the non dependent receives PIP too. I can't see this anywhere can someone tell me who is right? And if I am right how can I prove this to the DWP? Thank you.