It is certainly not illegal to report a drunk driver. Quite the opposite. You are performing a public service. You are not obligated to do so and should she get in an accident and hurt someone you cannot be held liable. However, the bar she frequents could very well be held liable (this is referred to as "dram shop liability") if she hurts someone as a result of drinking at that bar and driving. Either way, reporting or not reporting, you will not be exposing yourself to either civil or to criminal liability (assuming that you do not assist her by serving her alcohol). It is a public service for you to report her, however, and you should do so if you think she is impaired to the degree that she is endangering others by driving.
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