Alhamdulilah.
Don't assume that she will be able to get all thoughts of him out of her head instantly. Shaytan is there and with stuff like this, he will play with her mind. After a few days, weeks or months, it is not unlikely that she might break and want to know how he is doing etc. If they know each other in real life, it is going to be doubly hard for her, given that she will see him around often (I guess).
In light of that, don't leave her alone or wipe your hands with the issue. It is not completely over. Persist in being supportive of your sister and let her know that she must be strong if the shaytan strikes. Try and guide her into being a better muslim always, inshaAllah that will act as a barrier and make her feel guilty enough to stay away for good.