Joanna's death was probably one of the darkest moments in friends... even though people did die in friends, they were generally older people like estelle, heckles, ugly naked guy, nana etc. who conceivably died of natural causes. Joanna was a working-age woman and she didn't pass away in a painless fashion.
Why would she care about her boss? Joanna was a terrible person. She was willing to fire Rachel just because Chandler didn’t call her after a date, she handcuffed Chandler and left him stranded in her office. Not to mention she humiliated Rachel in front of superiors and made it seem like she was a terrible employee
She did feel bad at first then it dawned on her about her job. She is just a human being they weren't that close and she did humiliate her day before in the interview TBH she doesn't deserve much sympathy she was a horrible woman.