“They're a bad influence on him.”

You have a teenage son. Recently he's started hanging out with a group of friends who are rough and like to get in trouble. You think that he's recently started smoking because of these friends. You think to yourself:

They're a bad influence on him.

(someone) is a bad influence on (someone)

If you're a "bad influence" on someone, it means that you cause or encourage them to do bad things, or make bad decisions.

People usually talk about someone being a "bad influence" on other people of the same age (especially young people) and the same social status. 

"Bad influence" can be used in a couple of different ways:

Dolly was undoubtedly a bad influence on her youger sister.

Listening to that kind of music had a bad influence on them.

In addition to "bad", you can also "be" or "have":

a good influence

a huge influence

a positive influence

a negative influence