There is nothing in the ANO that says a pilot ceases to be a student as soon as they have passed their skills test.
Maybe not, but think for a moment:
A student signs up to gain his PPL. Once he has passed the PPL, the contract between him and the school where he was learning has been fulfilled, the student is therefore no longer a student (simplistically speaking although we know he could still learn more.) However you are going to continue calling him a student until his licence arrives.
If you are to deem this person to be a student, then - and here I think you will agree with me - the ANO does specifically state that a STUDENT PILOT (and this is what you are calling him) MAY NOT carry any passengers with him when solo.
Please note the original post was asking to carry a passenger with him hence my statement about legality.....
The insurance side of things is another aspect I was also pointing out and I would ALWAYS want to ensure that I am ensured when flying because the insurer might not take too lightly to a "student" who has passed his test being allowed to fly on insurance as a student rather than as a newly qualified pilot - to them, it's all about risk management and premiums.