In the case of something such as abuse, or suicide:
I would tell someone. Even if they never trusted me ever again, I would gladly sacrifice a friendship if it meant saving a life, or maintaining someone's well being. I mean, that would test the waters of said relationship.

In America, we have something called 'doctor-patient confidentiality (that may be practiced somewhere else as well, just covering my bases) and even if they suspect abuse of any kind, they have to let someone in authority know.