In the context you describe, the doctor could use either. They are both perfectly fine (and idiomatic).

Thanks Blake. Specifically, the context in which the sentences are used í in a hospital, and a doctor is talking to his assistant to let him know that "she" found that baby ( and now both she and the baby are in the hospital). So which sentence should I choose ?

It is just different tense. If it was in the past you would use the first one. If it was recent it would be the second but they are both correct.

She is the one who found that baby.