What are the UX implications of putting a web link in a mobile app’s navigation menu?

I've been asked to add a link to a third party website within the main nav menu of our mobile apps. We sell our core products within the app, but other fringe products are sold through a third party website. This sounds like a bad idea to me, but I don't have any concrete reasoning other than "web links don't go in the app navigation, because they aren't part of the app."

Are there specific issues we may be introducing if we proceed with this change?

Might adding some form of external link icon to the menu item suffice?