I've had 3 friends call me (at separate times) and tell me they were about to commit suicide. for 2 of them I called their husbands because it seemed like the right thing to do - 1 forgave me and we are still very close to this day. the other one did not and we don't speak anymore.
The third one lived by herself, across the country and was already halfway to an overdose, and i had to call the police in her state and give them her address and stay on the phone and try to keep her awake so she could answer the door when they got there. She thanks me still.

If you tell someone you are going to do this, I don't care about confidentiality, it is a dangerous situation and whoever you tell has the obligation to do the right thing which includes telling a significant other.