To be fair, he did acknowledge at the time that we initially discussed his flirty friend that he should have informed me sooner, and would do so in future. The main question at this point is how to manage the introductions, and the advice received thus far on that score has been very useful.
Regarding the chatting at night, it's not the only thing he does; our sleep schedules vary considerably anyway and he's a decided night owl. He spends a lot of time at night volunteering on an advice board and reading other discussion forums and such as well; it's not all flirtation, and my wording should have been clearer on that point. I don't want to paint an inaccurate picture of him or his activities.
While I would rather we had similar sleep schedules, it's not something that's likely to change much, but I do at least like to know in general what he's doing during that time, without making him feel that I'm policing him. The main issue there is with my own reactions and trust subscriptions, something I'll have to work on. ;-) And, of course, balancing our individual activities and schedules with time and attention spent on each other. I'm interested in hearing how other people manage such things, whether the extracurricular activities are of a poly nature or otherwise.