Girl asking a guy out on a date - pity or admire the girl?

My sister met this guy the other day at a bar who was a co-worker at some going away work party for her best friend. She says she talked to him for just a bit, before he left, and that he was 'handsome'. So then my sister finds out his work e-mail from her friend and asks him out a few days later. He wrote back saying thanks but he has a girlfriend already. Is my sister's behavior to be admired or pitied? I'm surprised that she would do this as she normally does not have much confidence, but then again maybe this behavior reeks of desperation, and someone who sees themselves as low-value? opinions?