She told me that I was “taking his side” and “defending him” and I should take “her side.” I explained that there isn’t anything to defend him for- he’s not a college roommate that wants to be a 3rd wheel to a date- he’s in his own apartment and watching tv.
Ugh!

I’ve been there too. Exact same response, even identical words.
Different circumstances. Same dynamics. What both you and I did was to explain the situation from the perspective of the person our spouses were attacking.
For me, it seemed only a natural thing to do. I tried to give my husband a context that he didn’t seem to understand about how his sister was responding. You tried to convey that this loud obnoxious guy has every right to be in his apartment as much as he wants.
Chalk it up to a great example of JADEing—you were not justifying, nor were you arguing or defending. You were merely EXPLAINING which is
not allowable 
when your BPD partner is upset.