。
She came on her own.
Literal
She [topic-は] alone [as-で] came.
A minimal frame for 一人で. Past 来た closes the sentence with 'came'; the speaker is at the destination, observing or commenting on her arrival. In context this often signals mild surprise — coming alone may have been unexpected, since Japanese social events frequently presume couples or groups (家族で, 友達と, etc.).