Am I sinful for helping a friend of mine – who breaks his fast in Ramadan due to the arduous nature of his work – by bringing him food?

If a worker or an employee finds themselves thirsty to the point where it incapacitates them from performing their work, they are permitted to break their fast. However, they are not allowed to pre-intend breaking the fast. What your friend is doing, by not fasting in Ramadan, is impermissible. Rather, it is obligatory for him to intend fasting every night. If fasting exhausts him, then he is permitted to break his fast. You must alert him to his error, and if he responds positively, you may bring him food if he is permitted to break his fast; otherwise, no, as that would be assisting in sin.