Learn Italian: La cancelliera ha parlato con la prefetta. - The chancellor f. spoke with the prefect f..

For professions ending in -iere, the feminine is formed by changing the final -e to -a. This follows the exact same pattern you learned previously for words like cameriere to cameriera or infermiere to infermiera.

Is prefetta standard? I thought some official professions ending in -o stayed masculine even for women.

Yes, la prefetta is the standard modern form. While historically many titles ending in -o (like il prefetto or il sindaco) were used for both men and women, modern Italian strongly encourages using the regular feminine -a ending for women (la prefetta, la sindaca). You might still hear the masculine used by older speakers, but the feminine is the modern grammatical standard.

Since both people mentioned are women, why is it ha parlato and not ha parlata?

The past participle parlato only changes its ending to agree with the subject when the auxiliary verb is essere. Because the verb parlare takes the auxiliary avere (ha), the past participle stays strictly in its default -o form, regardless of the gender or number of the subject.