As a user, assuming by "new window" he also means "new tab", I would always prefer external links to open in a new tab. I don't want to click a link half way through an article and suddenly have the new page replace my current one. Because hey, I was reading that.
If he is specifically talking about arguing against opening in a new window as opposed to a new tab then I'm behind him 100%. Why would you even do that? It makes no sense.
I would be interested in clarification however. And if he is arguing against opening new tabs, why?
After both your posts I just realised that yes, it is better to let it overwrite the page, because the option is there for the user to override this with the middle-click.
I've middle clicked all links for so long that I didn't even think about the fact I was choosing to open things in a new tab.
As you hint in your own response, it's a personal preference (and some browsers will even let you set it). I think it's a good convention that clicking a link replaces the current page, since it was established in the pre-tab/pre-multiple window era and thus follows the principle of least surprise. Besides, you'd be amazed at the number of users that don't know how to close tabs, so it's a bit more resource-friendly to abandon the last page (especially on tablets).
That said, I tend to middle-click links so they open in new tabs if I don't want to lose my place (especially here on HN), and I'm happy to have that option. I have no idea what tablet users can do in that situation. On my Android phone, I make heavier use of the back button, history and bookmarks compared to my desktop surfing.
If he is specifically talking about arguing against opening in a new window as opposed to a new tab then I'm behind him 100%. Why would you even do that? It makes no sense.
I would be interested in clarification however. And if he is arguing against opening new tabs, why?
Edit: Unless he is talking about a pre-tab era?