"Maybe he/she changed his/her mind, sigh."
Here's a breakdown:
* Siguro: Maybe
* Nagbago na: Changed (already)
* Ung isip niya: His/her mind
* Haizt: A sigh of disappointment or resignation
This sentence expresses a feeling of disappointment or resignation because someone has likely changed their mind about something. The "haizt" signifies this feeling.