DNA/Hair Testing and how far back can it go?

One of my friends is going through some court stuff with her baby daddy, the lawyer told her that they are going to do a hair follicle test to determine whether she is deemed fit to have sole custody of her son or not. Now we all know that most lawyers are serpents, but this guy said they could test as far back as 6 months, and me not believing anything a lawyer ever says told her it was a lie (Lol...) So my question is can they do that or not. I've seen answers on here where they said they can test as far back as your hair's life but is that true or not? Because she doesn't do drugs, she used to smoke bud but hasn't in a while. It wouldn't seem fair if they took her son away over something she used to do as a kid.