Hi, I'd like to help you with your question.
Your co worker should not be drinking at all. Alcohol is a depressant and if he is already depressed he is just making himself feel a lot worse. Alcohol can also affect his judgment so if he is feeling suicidal, he may be more willing to attempt to hurt himself if he is drinking.
Adding alcohol to anti depressant medication can be dangerous. Drinking alcohol with anti depressants can cause an increase in depression, an increase in side effects from the medication (if the medication causes you to feel tired, the alcohol would dramatically increase the effect), he can become even more intoxicated because the medications increase the effect of alcohol, and if he is taking a certain kind of medication called a MAOI, he can experience a dramatic spike in blood pressure and have a stroke.
If you can, talk with your friend about the dangers of using alcohol. He may not listen, but talking to him will let him know you care. That way, if he ever needs anyone to turn to in the future he knows you are there.
Let me know if I can help in any other way,