Background: I am a PhD student in my final year, and I'm writing my thesis now. My PI (not a professor) is in his late 40's and is generally kind in public. However, he is often moody, and I feel he stresses his students out by repeatedly making judgemental statements about them in the lab. We usually try to keep our cool and let him take his bad mood out on us. One colleague is a major target of his bad mood. This is probably because she is not always in a good health and asks for sick leave occasionally. My PI only allows us to take in total 4 weeks of leave every year (despite our university grants 8 weeks of leave in total). However, in my opinion see she works hard on her experiments day and night.
Situation: Last year, at a lab meeting, this colleague said that two of her cousins had COVID. My PI smiled and said 'Oh now I see the problem in your family gene.' There were 4 PhD students (including me) and a technician present. I was shocked and the others also seemed to be at least surprised. After the lab meeting, I asked the colleague if she was OK, and she smiled embarrassedly.
I would like to report this to the department. However, I wonder if this may be more of my colleague's choice. In my opinion, my PI has created a toxic environment for all of us, and I don't want to let this go. He makes micro-bullying or threatening statements occasionally, but this public humiliation seemed very serious and crossed a red line. We've been treated similarly but not as bad as this, so people seemed to accept my PI for who he is. However, recently my colleague is treated especially badly by my PI and she looks very frustrated. I feel bad for her and feel I need to do something.
Question: As this public humiliation happened about one year ago, how can I report this if I don't have recorded proof? Are there other choices to make our research environment healthier? What can I do? Thanks ahead for any help and suggestions.