dnd 5e – Can a sorcerer cast a spell during the first combat game after starting the fight with a ready spell?

My wife plays a magician. We hear orcs coming up the hall. The assistant decides to prepare a spell for opening the door. So she throws the spell and holds it until the door opens 10 seconds later and the trigger is triggered. BOOM the fireball starts. The fight begins Can she make the first round of combat, even though she has released the energy of the spell provided as a triggered reaction?

My first inclination is, YES, because she did not cast the spell at the beginning of the fight, she did that when she prepared the spell. She has just given her energy to the door opening as a REACTION, see page 193.