How do I remove Activation Lock on iPhone 8?

Hi my girlfriend. Left her iPhone 8 while she was visiting me in California, she said I can have it . She bought a new one . I know her phone number and she said it has activation lock she can remember password and account

What do I do to get activation lock off an iPhone 8 that my girlfriend gave to me , while visiting . ? She bought a new iPhone and her iPhone 8 has an activation lock ??!! She can’t remember password

[Re-Titled by Moderator]