Debbie downer coworker

How do you deal with employees who are, for lack of a better word, Debbie downers?

I am not a manager yet, I’m early career and I am just helping the PM with schedule and other things and I am invited to all the meetings, but I am curious what the appropriate way to deal with this would be as I’m not sure my boss is handling it well.

This employee questions every decision the PM makes and will not accept any explanation. They talk bad about the PM behind their back and are openly confrontational in meetings where I don’t think it is warranted. When we ask them to do things they say it’s not their job. They don’t bring issues to our attention, they complain to others until we hear about it through the grapevine and then when we try to help them they shoot down every solution without even considering them. It’s like they thrive on conflict and being angry.

I’ll admit morale on this project is low, but other people who have issues come at it with an attitude of hey, let’s work on this together, I know everything sucks but together we can try, while this coworker comes at it with an attitude of you’re wrong and dumb and I have no solutions. We’ve asked them to provide fixes for the issues they point out and they say it’s not their job or problem and then they just keep complaining.

It’s really exhausting and I dread even speaking to them now. Every meeting they are in turns into an argument and last week they spent 20 minutes yelling at me about how everything I was doing was wrong. I got overwhelmed and went and cried in my car for 20 minutes and after they found me and said I was doing a good job, they were just taking their frustrations out on me but never apologized, just said it was what I should expect 🥴

My boss basically rolls over to this person, they are a senior technical employee and losing them would make the project totally tank. There’s only a few months left so I think he’s just trying to ignore it but I don’t know how much more I can handle. What would you do?