I'm not assertive

So my Best friend has bipolar disorder. One week she will be okay, the next pissed off at everything, and the next on the verge of suicide. Her family couldn't afford her meds anymore, so she stopped taking them. She's VERY angry right now and we were at lunch and she asked me to save her a seat. A girl she doesn't like sat down and she saw her stuff and said she wasn't sitting there. I said nothing. She came back and stormed off and is honestly making me feel like a horrible person and all I can really say is sorry.