She looked surprised. Even if she'd run into several people now who were nice and did nice things for her, it was still a shock. Being a lab rat for years conditioned you to expect the worse out of people. There was a small hesitation even if her face probably gave away how much she wanted to say yes, then she nodded.
Then she smiled.
Then she looked a little guilty, then sheepish, and eventually she pulled out her phone and typed the words Thank You before holding it out to him.