I asked him about this again and apparently you nailed it. He said he'd been worried that he might have excess copper (he felt tired all the time and had some other symptoms - he had been a vegetarian for a long time) and that's why he was taking the zinc to begin with. When he saw the green tinge he figured the treatment was working and apparently his symptoms went away at the same time.

There might have been something else going on, I don't know. It seems at least mildly surprising that you could excrete enough copper to visibly affect urine color.