I've been circling around this issue, trying to break it down for a while now and there are a few issues I keep hitting.
- Putting a young girl's safety in danger is unacceptable.
- Her mother is the actual source of the danger.
- If it wasn't your son, there's a good chance it'd be another boy.
You're not confident that you have the facts of the situation.
(Before I go any further, it's best that I divulge that I'm a confrontational person who believes in trying to address problems directly. You might want to take a softer approach than me...)
Firstly, you shouldn't act without knowing the facts. Teenagers are notoriously prone to inventing drama, so tread warily. Speak to the girl and see if she corroborates your son's story.
If she doesn't, then life becomes much simpler. If she does, then it's time to tread carefully.
Parents who harm their children are usually doing so because they love them. They may express it in negative ways but often they're doing it because they believe that it's the best way to protect them from a real, or perceived threat. This is not to belittle or excuse the terrible things which are sometimes done, of course.
I would take push angle when initially speaking to the daughter. "What is your mother trying to protect you from? Is there a way that you can have what you want without incurring whatever risks your mother fears?"
The mother may be utterly opposed to her daughter's boyfriend but there may be a way to mitigate her worries, perhaps by making her more central to the discussion? Perhaps if she were to meet your son, she'd be able to see past his ethnicity?
Here's the thing -I- would do which is possibly a Bad Idea.
I would speak to the mother directly. First off, to try and put her mind at rest (if that were possible) but also to let her know that someone is paying attention. If she's considering extreme action, knowing that there are people watching might make her think twice.
Of course, it might also make her defensive and make things worse...
If you believe that there is a real threat, speak to the Child Protection Service (or local equivalent). Definitely follow their advice, rather than mine!
Also, if the girl is 17 years old as well, she will soon be an adult and able to leave home to start a new life. I know that a year is a long time in the life of a love-struck teenager but would it be possible to wait, to keep the relationship on a low-key footing until she's 18, at which point she can do what she likes? (As I write this, it feels like futile advice but it's technically possible.)
The last point, and it's one that never sits well with a teenager, is that everyone is in love with someone at age 18 but not many of these relationships last for very long. Making profound, life changing decisions on the basis of a relationship that may only last a few years is not always a good idea. I know that's a bit brutal but it doesn't mean that it's not true.
Anyway, I wish you all the best. "The course of true love never did run smooth"