It's easy to explain, but her language (Vietnamese) has no relation to English other than forced adoption of the Latin alphabet, so she wouldn't see that connection.

"Sorry" is most commonly translated as "xin lỗi" which literally means something like "request forgiveness". It's connected exclusively to the notion of fault, not sadness. The real issue is that sorry <-> xin lỗi is a ubiquitous but poor translation, because the meaning of xin lỗi is much more specific than sorry.

I speak Urdu (another South Asian language). If you asked someone the meaning of sorry in Urdu they would always say "ma'afi/ma'af karna" which is very strictly "asking forgiveness" although it "can" be used as "I didnt hear you / come again" literally nobody ever uses it that way