Translation:She smiled happily!
I'm native speaker.
For my feeling, it's because of the rhyme. “笑” is the verb exactly, but when we speak we say "笑了”. It makes the sentence sounds more smooth and more human being. If just say "她高興地笑” sounds like a robot smiles.
You can find some other one-syllable verbs have the same regulations. For example, “哭”, "睡". 他悲傷地哭了 He sadly cries. 他安詳地睡了 He falls asleep peacefully. Here, "了” is just the function words and meaningless, but makes the sentence smooth. If you don't add "了”, we still understand by all means, but it's not the way native speaker's talking.