Not a friend though. And someone like that can be very draining, which is the last thing anyone needs in their lives.
I'd try changing the subject, or tell her you're not comfortable about it, especially talking about other mothers with kids in the same class, but if all she wants to do is gripe constantly, then I would be hurrying away on various pretexts.
The last thing you want is to find that you are lumped in with her, Negative Nancy, in other people's minds, iykwim.
PS I have no kids but I believe that it can be a tightrope betimes at school gates.
So please feel free to ignore all of the above