Emphasize the country not the city when referring to a person's nationality. Even though she's from Porto, you should say she's Portuguese, not Porto-ese.
Pro Tip 2/3
Not Gendered
Use 'Portuguese' for both male and female people from Portugal. Both my mother and father are Portuguese.
Pro Tip 3/3
Cultural Sensitivity
Be aware that cultural identity is important for many Portuguese people. She's proud to be Portuguese and celebrates traditional festivals.