I think it depends on 1-how close you are and 2-the reason you feel she shouldn't take it.
There are an awful lot of variables.
If, for example, you were to tell me that this is a close friend who has some baggage dealing with harsh, misogynistic people and you happen to know that the boss she'd be working with is a *insert expletive of your choice here* chauvinist with severe anger management problems; I would tell you that giving your friend enough information to make an informed choice would be a kindness and is what a good friend does.
If, on the other hand, you tell me this is an acquaintance and you think that the new time constraints of the position will make it impossible for her to dedicate enough time to her volunteer work/hobbies/kids/whatever, I would tell you to MYOB.
To many variables.