This is the 194th most frequent Vietnamese word.
ấy
"Ấy" is a pronoun in Vietnamese used to refer to a third person (he, she, it, they) or to show closeness, often context-dependent.
In this sentence, 'ấy' is used as a demonstrative pronoun referring to a specific person (your friend) who is implied in the context.
Bạn ấy rất chăm chỉ học tập.
That friend of yours studies very diligently.
Here, 'ấy' is used with 'cô giáo' (teacher) to demonstrate a specific teacher being referred to, indicating familiarity or previous mention.
Hôm qua tôi có gặp cô giáo ấy.
Yesterday, I met that teacher.
In this context, 'ấy' is used temporally to refer to a specific day that has been mentioned or is understood in the conversation.