Thank you for sharing!
I think this points to the fact that, even if people keep their poly relationships "separate", things can't help but spill over into the other relationships in some circumstances. (Not saying that your relationships were separate, just saying that each person is a whole person and who they are with and their other relationships are part of the whole "them")
For me, MrS and I had a huge fight one day (not over a poly-topic), poor Dude got caught in the cross-fire - I was so wrapped up in my marital drama that I (unfortunately and wrongly) told him to "get out" - I couldn't deal with his stuff and my stuff with hubs at the same time. Lucky for me he negotiated to stay and was really instrumental in me and MrS figuring our shit out.
As to taking it harder than the two involved people - it's ok, really. You all had good thing going on, your poly bubble got burst - it's ok to mourn. Hopefully you can support them both through this time - who else is in a better position to do so?
Not advice, just support.
Me: poly bi female, in an "open-but-not-looking" V-plus with -
MrS: hetero polyflexible male, live-in husband (24+ yrs)
Dude: hetero poly male, live-in boyfriend (5+ yrs) and MrS's BFF
SLeW: platonic girlfriend and BFF
Lotus: "it's complicated"
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe