Several Ideas
* Tessa can transform into a swarm of bugs. Maybe she can "detach" a few bugs as spies
* Tessa could have compromised some of Nic's minions. They cannot speak, but they can write, email, text, sign language or any number of other options. Do Minions Twitter? Tessa has been rivals with Nic for centuries. She would certainly have put some effort into spying on him.
* She could have hired people to spy on Nic using non-magical means
* She could have some method to follow/observe her daughter - who was -- after all - at all the sights of attack. Blood ties allow scrying.
* Who says that Fallen would not have divided loyalties or otherwise have differing agenda. Lasciel was clearly involved and she was notorious for not being a "loyal minion" of anybody. After all, all the interventions were focused pretty hard on stopping/hurting Harry - a prime objective for Lasciel.