The proper etiquette is to use their current name. Using their birth name / given name after they have transitioned and chosen a new name is called dead naming. Dead naming is often used by transphobes who purposefully intend to be hurtful and invalidating to a trans person, but it can also happen to be used by people who may slip up on accident simply because they haven’t fully processed the acknowledgement of the person’s transition, which can take time, particularly if you’re not directly involved in their personal life. Most trans people understand that mistakes happen, which is why It’s important to apologize sincerely but briefly, when accidentally using their dead name, and then make every effort to refer to them -even their past self- using their current name.