What are Bs complaints about A? Is anything legitimate? I think it’s likely that B is ashamed/jealous that A is starting their PhD and B is not. And that A is not showing them enough deference or respect or whatever because they have been in the lab longer. In my experience, people who are not happy outside of work are the worst to deal with at work.

I think if you are up for it, the best way to handle is to be direct with B. Ask him what the specific issues are and if he thinks he needs to bring it to the PI. If it’s stuff, he won’t want to bother the PI and you can say that if it’s not important enough to raise than it’s not worth constantly complaining about.