She told you for a reason. Sometimes that kind of thing is because the person telling it wants someone else to take action because they can't or won't. As far as supporting her, do you want to? If so, call a local (to you) women's centre, hospital or government medical clinic and ask for advice on that. Should you call the police in her town and have them investigate? I would do that but there will be a cost: She may become angry even if she subconsciously wanted someone to do something. Would that bother you?
You know her, you know how old she is (if she's a minor DO call the police or her local CPS regardless). They can do an investigation but will not reveal where the information came from. Your ex would likely figure it out so you are going to have to decide what to do as far as reporting it.
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A Parent, old geezer, and occasionally right. Good judgment comes from bad
experience, and a lot of that comes from bad judgment.