At least in Travis and surrounding counties, I haven't seen any urine tests for alcohol at probation. As you stated, they typically only administer a breath test. In your situation, they may still test your urine for other substances just to cover all their bases. You'd probably be better off not making any additional statements to your PO about the details of your PI arrest. I think you'd rather be in a position to argue that the cop arrested you for PI because your friend was a jerk to the cop and the cop wanted to try to undercut your credibility as a potential witness for your friend on his DWI. Instead one is left wonderng exaclty how much ethanol passed from your frinend's mouth to yours that the cop could smell it as strongly as he will undoubtedly say he could.