Should he sit with his colleagues at work when they are drinking alcohol?

Allah has favoured this ummah over other nations in many matters, chief among which is enjoining what is good and forbidding what is evil. Allah says (interpretation of the meaning): 

“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al‑Munkar (polytheism, disbelief and all that Islam has forbidden), and you believe in Allah”

[Aal ‘Imraan 3:110]

What you have to do – as you are the only Muslim in your company, as you say – is to be proud of the teachings [?] of your religion, and strive to apply them, and not to do anything that you are forbidden to do. This is something that will raise you in status and increase your reward. Staying with them even though you are not drinking alcohol yourself is still a sin in itself, because Allah has commanded us not to sit in places where evil is committed, otherwise what befalls those who are committing the evil action will also befall us. 

Allah says (interpretation of the meaning): 

“And it has already been revealed to you in the Book (this Qur’aan) 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”

[al-Nisa’ 4:140] 

“And when you (Muhammad) see those who engage in a false conversation about Our Verses (of the Qur’aan) by mocking at them, stay away from them till they turn to another topic. And if Shaytaan (Satan) causes you to forget, then after the remembrance sit not you in the company of those people who are the Zaalimoon (polytheists and wrongdoers)”

[al-An’aam 6:68]

The Prophet (peace and blessings of Allah be upon him) said: “Whoever among you sees an evil action, let him change it with his hand (by taking action); if he cannot, then with his tongue (by speaking out); if he cannot, then with his heart (by hating it and feeling that it is wrong), and that is the weakest of faith.” Narrated by Muslim, 70. 

This denunciation of the heart refers to the stress and agitation that one feels in the presence of evil. This is an individual obligation upon all people in all situations, and they have no excuse for not doing this, because no one has any power over the heart. Staying in a place where evil is being committed goes against this required denunciation. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said: 

The believer has to fear Allah with regard to His slaves, but their guidance is not his responsibility. This is what is meant by the verse in which Allah says (interpretation of the meaning): 

“O you who believe! Take care of your ownselves. If you follow the (right) guidance [and enjoin what is right (Islamic Monotheism and all that Islam orders one to do) and forbid what is wrong (polytheism, disbelief and all that Islam has forbidden)] no hurt can come to you from those who are in error”

[al-Maa’idah 5:105]

Following right guidance means doing all the obligatory duties. If a Muslim does his duty of enjoining what is good and forbidding what is evil, as well as other duties, the misguidance of those who are misguided will never harm him. This may be done sometimes with the heart, sometimes with the tongue, and sometimes with the hand. Denunciation with the heart is obligatory in every situation, because no harm can result from it, and whoever does not do that is not a Muslim, as the Prophet (peace and blessings of Allah be upon him) said: “And that is the weakest of faith.” 

Majmoo’ al-Fataawa, 28/127 

Moreover the Prophet (peace and blessings of Allah be upon him) forbade sitting at a table where wine is being drunk. 

It was narrated that ‘Umar ibn al-Khattaab (may Allah be pleased with him) said: The Prophet (peace and blessings of Allah be upon him) said: “Whoever believes in Allah and the Last Day, let him not sit at a table where wine is being drunk.” 

Narrated by Ahmad, 126; classed as saheeh by al-Albaani in Irwa’ al-Ghaleel, 7/6. 

See also the answer to question no. 8957; 6992. 

Finally we remind you of the words of Allah (interpretation of the meaning): 

“And whosoever fears Allah and keeps his duty to Him, He will make a way for him to get out (from every difficulty).

 And He will provide him from (sources) he never could imagine. And whosoever puts his trust in Allah, then He will suffice him. Verily, Allah will accomplish his purpose. Indeed Allah has set a measure for all things”

[al-Talaaq 65:2-3] 

So give up these evil things, and those trips and sitting with those people. Seek the reward for that with your Lord. If this leads to your leaving your job, then you will have a great reward with Allah and you will find goodness, a way out and provision by His Leave. 

And Allah knows best.