How did she know he'd changed his name when she saw him again by chance?

A few days later, he sees her by chance when having dinner with his two close friends, and they have a short interaction.

I'd only seen her for a few seconds before that, by chance.

Joseph knew that now he would not see her again except by chance.

I don't believe you saw the bird just by chance.

Does one get to see such things by chance?

People who saw them by chance did not know their meaning.

We watched and waited, then saw him again almost by chance.

Ms. Wang later saw a copy of the appeal decision by chance.

The thing they saw by chance, suddenly changed the whole situation.