Is sitting with non-Muslim coworkers who are drinking alcohol a sin, and how can one atone for it?

The Prophet, peace and blessings be upon him, forbade sitting at tables where alcohol is consumed, saying: "Whoever believes in Allah and the Last Day, let him not sit at a table where alcohol is drunk." This is because a believer must refrain from sitting in places where Allah is disobeyed, out of fear of incurring Allah's wrath. The Ahadith are explicit in cursing alcohol and those who deal with it, and one who sits there may fall into something greater than mere sitting. The prohibition is clear in preventing the believer from sitting at such tables even if he does not drink, as Allah Almighty says: "And it has already come down to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they engage in another conversation. Indeed, in that case, you would be like them." The expiation for this sin is repentance, seeking forgiveness, and not returning to such gatherings.