I have teenage boys in high school and hear all kind of stories about different crowds (from them). And, no, it’s not a loshon hora. Boruch Hashem, we enjoy open relationship and they know I will not judge them (or their friends) whatever they would tell me. They actually occasionally come to me for an advise on how to deal with this or that social, spiritual or other issue – the fact that many parents find hard to believe.
Now, we all want to be the authoritative figure in our children’s lives. What we don’t realize is that we don’t have to distance ourselves from them to achieve this result. We are already in a position of power as we were since their birth. As we will be until they will be able to provide for themselves.
My advice to your friend is to BEFRIEND her son, show her willingness to be his best friend. Only from a position of trust she can hope to help him. As his friend she should find it easy to ask him, what is it that he likes about these boys, what do they do when they “hang around,” listen without judging (and remember, judging can be in your facial expression, not only in words) explain to him why she considers these kids to be “at-risk,” openly discuss the fact that his choice of friends bothers her.
This advice is general. I would need to know much more about the family, the age (or is it ages) of the boy (boys) to say something concrete.