This happened to me. I told a very close friend and co-worker when I was really ill with depression that I was suicidal and had been self harming. She took it upon herself to go straight to my manager and tell him everything. He in tow went to HR. I was on a day off and got a voicemail from HR saying we had a meeting booked for the following week regarding a situation that had arised. Followed by a voicemail from my manager who said my friend had told him everything and he was concerned. I liked him so I wasn't mad to begin with. By the following week I was livid at my friend and him and HR. They were rather patronising. In the end I was sacked on ill health grounds which was BS! I never spike to my friend again. Sad times.