I can't speak from experience obviously, but I do know that love comes to people from unexpected directions - and although their rational mind is telling them that this is wrong, and won;t work, and they will be hurt in the end, they proceed along because the heart wants what it wants.
The feelings must be as individual, complex, and diverse as the people involved, so there is no template to follow for these situations.
Some people enjoy the variety, the excitement, the elements missing from their own relationship, the sheer novelty of someone else, but it takes a huge number of circumstances, none of which are controlable by the individuals, for everything to knit up neatly at the end and finish with happiness.
If you want to know how your friend feels, talk to her. Some people like to discuss, some people like to be secret - again, it's down to personalities - no rules.