Is there any indication that she feels the same as you or does she look at you as just someone who is a friend that she can confide in. Personally if she feels the same as you and wants to be with you go for it. Life is very short and I suspect that this will still be troubling you in 30/40 years time if you don’t at least find out how the land lies.If in her mind your relationship is purely plutonic you also have your answer. The reasons you think she stays with him maybe way off the mark, she may be still madly in love with him. I think you need to know her true feelings and take it from there. Everything else then can be worked around regardless of how unpleasant some of it may have to be.
Just to add my brother-in-law did exactly the same thing and they ended up together and Helen God bless her past away from cancer three years ago. Apart from losing his best friend he maintains to this day he did the right thing