What a sad situation. It is very tempting when a friend loses a partner, through death, divorce or the partner walking out, to point out the downside of the relationship in the hope of cheering them up. But even if you wait for a few years, it rarely helps, because it either takes away the veil of their imaginery perfect life, or reinforces niggling doubts about the imperfections, or lets them know that others have seen through the facade they built up.
OP's friend cut herself off from those who criticised her and stuck to the one person she thought understood and accepted her and her partner. Now this illusion is shattered she may be devastated and find it hard to live with. I hope she is attending grief counselling or has some other form of support.
I feel so sorry for OP too, she meant well and it is something many of us might be tempted to do. Now she has lost a good friend. Perhaps if she writes and explains that she thought whar she said might help her friend get over the grief, and realizes it was a mistake, apologizes, and then goes on to talk about the man's good points and she understands how much her friend is missing him, perhaps referring to some of the good times they had, and laughed about with him, the friendship may be recoverable, though it might take time to get back to what it was.