Learn Portuguese (Portugal): Ela trabalha com o pai.

Trabalhar (to work) is a regular verb that ends in -ar. To match it with ela (she), you drop the -ar from the end and add -a, making it trabalha. You use this exact same -a ending for ele (he) and você (formal you).

The English translation says "her father", but the Portuguese just says o pai (the father). Why?

In Portuguese, when talking about family members or body parts, you often just use the word for "the" (o or a) instead of a possessive word like "her" or "his", as long as the context is clear. Because the sentence starts with Ela (She), it is naturally understood that o pai means her father.

Could this sentence also mean "She is working with her father"?

Yes! In Portuguese, the simple present tense like ela trabalha can mean both a general routine ("she works") and an action happening right now ("she is working").

What if she was working with her mother? How would the sentence change?

The word for "mother" is mãe, which is feminine. Because of this, you would need to change the masculine "the" (o) to the feminine "the" (a). The sentence would become: Ela trabalha com a mãe.