Would you say something?

Yes. I would say "work it out" or "get out of there". Someone else out there is probably right for each of you

I was in a similar situation , but I just asked her to forget about the past and quickly changed the topic.

would not say anything and let him or her try to learn

There is a Chinese saying which carries the meaning that "A speech will either prosper or ruin a nation." Many relationships break off because of wrong speech. When a couple is too close with each other,we always forget mutual respect and courtesy. We may say anything without considering if it would hurt the other party. A friend and her millionaire husband visited their construction site. A worker who wore a helmet saw her and shouted,"Hi, Emily! Remember me? We used to date in the secondary school." On the way home, her millionaire husband teased her, "Luckily you married me.Otherwise you will be the wife of a construction worker." She answered ,"You should appreciate that you married me. Otherwise, he will be the millionaire and not you." Frequently exchanging these remarks plants the seed for a bad relationship. It's like a broken egg - cannot be reversed.

Would I? Well actually I usually do. I occasionally take a little heat for it but I've never lost a friend because of it either.

I would especially if i see that she isnt happy anymore with that relationship.people in that kind of situation sometimes need encouragement from friends for them to realize that being in a relationship like that will slowly destroy a person..One has to wake up that true love is where no one gets hurt physically or psychologically.