All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The mere difficulty of work does not allow one to break his fast, and neither does thirst. This is because fasting normally involves hardship, including thirst. Hence, it is not permissible to break the fast because of feeling thirsty or engaging in hard labor. Therefore, your friend is required to make the intention to fast; if he becomes thirsty, like normal, then he is not allowed to break his fast; however, if he gets unbearably thirsty and fears that he might die, then he may break his fast and make up for that day later on.
Allaah Knows best.