The money a taxi driver gets in return for driving some of his passengers to their destination is his payment for his job and it is lawful. 

If a Muslim takes part in drinking alcohol with the drunkard at the bar, he is a sinner. Thus, it is better for him to stay away from the likes because helping in a sin is also a sin.

However, if he was compelled to convey these drunkards without actually participating in their filthy act, there is nothing wrong with that.

The most important thing is that he should not be affected by their behavior let alone fall into what they are into because bad deeds are easy to emulate than good deeds.