Does he have a visitor visa or a visitor record? If it's not valid for re-entry then I would assume it's a visitor visa. What that means is that there is no guarantee that he will be allowed to re-enter, but it also does not mean he will be refused. It simply means that as long as he doesn't leave the country he is legally allowed to stay as a visitor for the period indicated on the visitor record. I often crossed the border with a visitor record in my passport while awaiting my PR, and I found it facilitated re-entry. The border guards did not feel the need to question me about my visit, since I had already discussed my visit with another border guard in order to get the visitor record. Also, I suppose they could (should?) technically reissue a new visitor record every time you leave and re-enter, they normally don't. So the visitor record he has now will remain valid if he leaves and is granted re-entry, and he will still have to respect the time limit listed on the visitor record. In other words, there is no extension of your time as a visitor just because you leave the country and re-enter.