He didn't even notice the first time she healed him.
The first time he noticed Yuna healing him, he'd fallen back from the fight, trying to catch his breath. Okay, so he had to prop himself up with his sword, but Lulu and Wakka were back to back now, pretty much surrounded. One of the fiends - a flying thing with a beak like a sword - took a dive for the black mage, and Wakka knocked it out of the sky. Lulu managed to fry it as it dropped, but something was trying to barrel past Kimhari and - and he couldn't just stand here!
He'd managed to sway to his feet, but nearly went straight back down. Out of the corner of his eye, the blond blitzer could see something glowing, and he jerked his head round, trying to bring his sword up -
Yuna smiled at him, gesturing with her staff, and the light that had been building around her rushed into him. He gave her his best "Whuh?" look - but Wakka grunted and Tidus flew back into the fight.
He noticed after that, which probably wasn't a good thing. Tidus' reactions to having magic used on him wavered between that's so cool, and the skittishness Wakka showed when it came to machina. Every time Yuna healed him, he had to turn and look at her. Acknowledge that he knew she'd done it.
It wasn't until she managed to heal them all at once that Tidus realised he wasn't the only one who did it.