YC already knows the dangers of "co-founders" who don't have a working history beyond getting together for this startup. Technically, it should have filtered her out of the application process.
I didn't mean it absolutely even though I wrote it that way, obviously there are many other factors and exceptions, but this is one factor that would work against a person.
As for indicators, I think that people who have worked together for a shorter amount of time are more likely to break up. This is because fights and disagreements between co-founders are inevitable, and in startups they are more intense compared to regular business working relationships. When you have history with your cofounder, then you've likely survived those fights and are more likely to get through the ones coming.
