Translation:I know that her boyfriend is hiding here.
"Núp" is usually used in the South. "Nấp" is used in the North. "Giấu" is not like the 2 above, it is more like to hide sth or sb, not to hide yourself. But you should use the word "trốn" in this sentence, it is more official and more formal than "nấp" or "núp". You don't need to remember the two above, it is rarely used. "Trốn" is a safe way for all dialects and all situations.
Btw, if you combine the verb "chạy" with "trốn", you will have "chạy trốn"! (escape)
Im Vietnamese, sorry for my bad English. Chúc bạn học tốt!