This question reminds me of a conversation I once had with my Grandfather before he passed.....he asked me what I would do if I found out someone I cared about was having an affair, his answer was that you do nothing and stay out of peoples lives, regardless of what they may mean to you.
I could see his logic as I always could, but the policy of non-interference is good only for ourselves, it may be valid but it's ultimately self-serving, and that's not what we ought to aspire too.
I would tell the person in question, but that is all I would do, I would not interfere by way of telling them they should walk away or slow things down etc....I would simply provide them with the information they have a right too and that they need so they are able to guard their heart in the way everyone should always be able too.
In my opinion I wouldn't be much of a friend if I let someone I cared for go into a relationship missing a serious piece of information, or simply my honest opinion about what they are getting into, true relationships can survive that kind of thing.