Clara, I would tell her, as sooner or later the truth will come out and someone else will. Then she will only wonder what you knew and for how long. My grandmother killed herself in 1901, also with a razor, and though I knew there was a mystery I only discovered it in 1997. My late father, who was also a WW1 veteran and never talked about the war, had walked past the house where she killed herself every day on his way to and from work for well over 40 years, and had never mentioned the suicide to me or my brother, though when I told her it was obvious that my mother had known about it.