Praise be to Allah.
What is prescribed in your case is to exhort him and explain to him the seriousness of what he is doing by breaking the fast in Ramadan, which is one of the gravest of major sins.
In addition to that, there is another major sin, which is blatantly and openly committing this major sin, regarding the matter as insignificant, and not concealing his sin, which indicates that he has little respect in his heart for this obligatory act of worship. In addition to that, his actions may cause others to do similar things, or upset the believers and cause their enemies to gloat.
In as-Saheehayn it is narrated that Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “All of my ummah will be fine except those who commit sin openly, and it is part of committing sin openly for a man to do something at night, then in the morning when his Lord has concealed him he says: O So and so, I did such and such last night, when his Lord had concealed him all night, but in the morning he discloses that which Allah had concealed for him.” Narrated by al-Bukhaari (5721) and Muslim (2990).
So how about one who commits that deed openly, by day, and does not feel ashamed of it or try to conceal it?
With regard to how to advise him, undoubtedly what is appropriate for someone like you, who has no authority over this driver and others like him, is to speak to him gently when advising him, remind him of Allah and tell him to fear his Lord, may He be glorified and exalted, and make him understand the seriousness of what he is doing. Believing in one’s heart in the Lord of the Worlds requires a person to respect Him, and to respect His commands and adhere to them, and to respect His sacred limits and avoid transgressing them. Allah, may He be exalted, says (interpretation of the meaning):
“That [has been commanded], and whoever honors the sacred ordinances of Allah – it is best for him in the sight of his Lord. And permitted to you are the grazing livestock, except what is recited to you. So avoid the uncleanliness of idols and avoid false statement,
Inclining [only] to Allah , not associating [anything] with Him. And he who associates with Allah – it is as though he had fallen from the sky and was snatched by the birds or the wind carried him down into a remote place.
That [is so]. And whoever honors the symbols of Allah – indeed, it is from the piety of hearts”
If none of this works with him, and you see that he is ignoring it or is taking the sacred limits of Allah lightly, then what is prescribed in your case is to shun him, and refrain from speaking to him or dealing with him; you should not greet him or return his greeting, especially at the times when he is openly committing that great sin. So it is not permissible for you to sit with him when he is doing that, until he gives that up and repents from it.
Rather you should only deal with him as much as is necessary, as may be required by your workplace.
If you fear that shunning him may result in harm to your religious commitment or to you, because you are living in a country dominated by disbelievers, and you think it likely that harm may result from that, then in that case there is no blame on you for showing kindness to him to the extent that will ward off harm from you, whilst denouncing and disliking his evil deeds according to what you are able to do.
Please see also the answer to question no. 83581.
And Allah knows best.