How about why didn't she just walk away from the friendship when she realized she was developing feelings for this married man? 

She justified it by saying she "didn't want to lose her best friend." So this was the alternative, I guess. 

Look, all relationships can be tricky to navigate and can end. But this is a relationship AND a lifestyle choice, so the logic, if you're trying to get people on board needs to be pretty sound and yes, a cohesive explanation about why this was the only possible choice she could make would certainly be helpful in this situation.