Learn French: Son amie est arrivée hier. - Her friend arrived yesterday.

Because the verb arriver uses être (to be) in the past tense, the past participle must match the subject. Since amie (friend) is feminine, we add an extra -e to make arrivée feminine as well.

If amie is feminine, why does the sentence use the masculine word son instead of sa?

You are right that sa is the normal word for feminine nouns! However, because amie starts with a vowel, French uses son to make it flow better when spoken. Saying sa amie would awkwardly crash two vowels together.

Does son amie mean 'his friend' or 'her friend' here?

It can actually mean either! In French, possessive words like son match the noun they are attached to (amie), not the person who owns it. The English translation says 'Her friend', but without that context, the French sentence could just as easily mean 'His friend'.

Why does this sentence use est instead of a to talk about the past?

Most French verbs use avoir (to have) in the passé composé, but a specific group of verbs uses être (to be). These are usually verbs related to motion or a change of state. arriver (to arrive) is one of these movement verbs, so it pairs with est.