This is a difficult question. 'Specially for me, 'cos I have been 'the other woman', even though the relationship never became physical, due to the fact that both of us had/have strong moral beliefs. But we loved each other deeply.
I was separated from my husband, but he was married to a very good woman; a very good wife - but for one thing: she had little or no interest in sex.
I liked his wife. We were friends. She knew we loved each other. But she made no effort at all, that I ever saw, to intervene. I kept wanting to tell her that she needed to be more giving sexually, and everything would be ok for her and him and that he would soon lose interest in me if she was more interested in him - but I lacked the courage.
The main reason most of us fail to tell people things they need to know is lack of courage (IMO), rather than anything else.
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Smile! Life is too short for bitching
Fljotsdale