This is the 10903rd most frequent Russian word.
выпускницей
A female graduate or alumna.
In this sentence, 'выпускницей' is used in the Instrumental case, showing the role or status related to oneself, 'being a graduate'.
Она гордиться тем, что является выпускницей университета.
She is proud to be a graduate of the university.
Here, 'выпускницей' in Instrumental case indicates what someone has become after completing a certain stage of education.
Моя сестра только что стала выпускницей.
My sister has just become a graduate.
In this example, 'выпускницу' is used in the Accusative case to denote the direct object of the verb 'congratulated,' referring to the graduates.