Just an update and temporary resolution for anyone who cares~
I spent some time last night talking to my friend about things and it seems that for now we're just going to keep this casual--or to be more blunt, we'll be 'fuck buddies', which I'm perfectly okay with. Honestly that's what I was expecting in the first place, so I'm quite content with the situation. If it happens to turn to something deeper later, great, if not, that's alright too. I'll at least have gotten a new experience and will be able to keep up a long relationship with these two.
He still hasn't told his partner, but that's because he's been snowed out of town and communication in general has been very limited. Not to mention this isn't really something to be discussed over the phone, in my opinion. He returned today though and when I left I was assured by the friend I was intimate with that he would talk to him about it. I'm quite confident that things will go well between them, so now I suppose all that's left is to just see where this relationship takes me.
Thanks again to everyone who took the time to read and respond to this. It's helped me a lot and now that I've gotten over my worries, I'm excited to jump in and have an adventure!