Is it ever OK to link to an external site (opening in a new page) directly from a main nav button with no notice? Both sites are owned by the same client, each focused on different aspects of their business.
You should try not to violate the Least Surprise rule. Having an external link mingled between internal links may lead to confusion.
It's OK if you have the external link marked in some visible way. If there's no room for icons next to links, you might try to play with different color or underline styles, or show a tooltip when hovered on the external link.
To further emphasize the point @fri was making its generally not a good idea to have navigation do anything but navigate within the same site. This is a behavior users typically expect when using a sites navigation. Along with expectations like same look and feel throughout. When navigating to an external site it breaks that expected behavior and confuses the user.
An acceptable behavior would be to put external links into their own menu labeled as such etc. which was again mentioned by @fri
Hope this helps.