My initial reaction would be that it would be highly inappropriate to attend someone's defense unless explicitly invited, and you should never ask someone for and invitation to anything. It should be something very private.
However, it referred to as a "Final Public Oral Examination" and the time, date, and location (i.e. Zoom link) is shared by the department and the university. I have been told that some things I would find mean and offensive are actually supportive and kind things to do, so I do not know if my initial reaction is the "correct" one.
I want to be supportive, while not going where I am not wanted/should not go. At the same time, I am not sure if there is just some level of unspoken assumptions/norms that I am unaware of.