How should he deal with a work colleague who reviles Allah and His Messenger?

Firstly: 

What the Muslim must do is avoid being present in any gathering in which Allah is being disobeyed, so how about a gathering in which Allah and His Messenger are being reviled? 

Allah, may He be glorified and exalted, says (interpretation of the meaning):

“And when you (Muhammad (blessings and peace of Allah be upon him)) see those who engage in a false conversation about Our Verses (of the Quran) by mocking at them, stay away from them till they turn to another topic. And if Shaitan (Satan) causes you to forget, then after the remembrance sit not you in the company of those people who are the Zalimoon (polytheists and wrongdoers, etc.)”

[al-An‘aam 6:68]. 

If anyone finds himself in a gathering in which people cast aspersions on the verses of Allah, or mock them, or show disrespect towards them, or revile them, then he must leave their gathering until they begin to speak of something other than the falsehood in which they were indulging. 

Allah, may He be glorified and exalted, says (interpretation of the meaning):

“And it has already been revealed to you in the Book (this Quran) that when you hear the Verses of Allah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them. Surely, Allah will collect the hypocrites and disbelievers all together in Hell”

[an-Nisa’ 4:140]. 

Al-Qurtubi said:

Anyone who sits in a sinful gathering and does not object to them will bear an equal burden of sin with them. They should be denounced if they speak sinful words or do sinful actions. If he is not able to denounce them, then he should get up and leave so that he will not be one of those included in this verse.

End quote from al-Jaami‘ li Ahkaam al-Qur’an (5/418) 

Based on that, what you must do is denounce this evildoer; if you are not able to do that then you must leave this gathering. If that is not possible, then in that case you must stop working in this place, so long as you can hear someone reviling Allah and His Messenger and you are not able to denounce it or leave that gathering. 

Shaykh ‘Abd ar-Rahmaan as-Sa‘di said: This prohibition applies to the one who sits with them and does not feel the fear of Allah when he joins them in haraam words deeds or keeps quiet and not denounce them. If he does fear Allah, may He be exalted, by telling them to do what is good and forbids them to do evil or to say these words that they are uttering, and that results in removal or reduction of the evil, then there will be no blame or sin on him.

End quote from Tayseer al-Kareem ar-Rahmaan (1/260) 

See also the answer to question no. 65551

Secondly: 

The best way to stand up for Allah and His Messenger in this situation is by doing several things:

 1.

Denouncing him for what he says and being harsh towards him in that regard; explaining the seriousness of what he is saying and the stern warning against reviling Allah and His Messenger; and advising the people around him not to listen to him because of the seriousness of what he is saying. 

The one who reviles Allah and His Messenger is an apostate from the religion of Islam; all his good deeds are rendered invalid, his marriage contract is annulled and his Muslim wife becomes haraam to him. If he dies in that state without having repented, then he is not to be washed or shrouded, the funeral prayer is not to be offered for him, and he is not to be buried in the Muslim graveyard. Similarly, he cannot inherit or be inherited from, and his fate in the Hereafter is to abide forever and eternally in the fire of Hell. 

See the answer to questions no. 22809, 114779 and 14305

2.

Ongoing shunning and severing of ties with him, if he does not repent from the disbelief and misguidance that he is following, as Allah, may He be exalted, says concerning Ibraaheem (peace be upon him):

“Indeed there has been an excellent example for you in Ibrahim (Abraham) and those with him, when they said to their people: "Verily, we are free from you and whatever you worship besides Allah, we have rejected you, and there has started between us and you, hostility and hatred for ever, until you believe in Allah Alone…”

[al-Mumtahinah 60:4]. 

Shaykh ‘Abd ar-Rahmaan al-Barraak said:

The least that must be done, in addition to denouncing him verbally and highlighting the fact that this is apostasy, is to cut off ties with him and not sit with him. If a person is faced with having to deal with him in the workplace, then he must shun him and turn away from him; so he should not greet him or interact with him in any friendly manner, and he should not speak to him any word that is indicative of lenience, so that he will feel the true nature of his crime and the abhorrent nature of what he has been saying.

End quote from Majmoo‘ Fataawa al-‘Allaamah al-Barraak (1/8) 

You have to advise the boss that if he is a Muslim and truly loves Allah and His Messenger, then he must be angry for their sake and stand up for them. The least he should do is to advise this evildoer, and if he does not refrain from his disbelief then the least that he can do is to dismiss him from his job; he cannot let love of work and worldly gain take precedence over love of Allah and His Messenger. Allah, may He be exalted, says (interpretation of the meaning):

“Say: If your fathers, your sons, your brothers, your wives, your kindred, the wealth that you have gained, the commerce in which you fear a decline, and the dwellings in which you delight are dearer to you than Allah and His Messenger, and striving hard and fighting in His Cause, then wait until Allah brings about His Decision (torment). And Allah guides not the people who are Al-Fasiqoon (the rebellious, disobedient to Allah)”

[at-Tawbah 9:24]. 

Whoever gives up something for the sake of Allah, Allah will compensate him with something better than it. So it is certain that if the boss dismisses this evildoer from his job, Allah, may He be exalted, will compensate him with something better than him, and his business will not incur loss because of that. 

And Allah knows best.