I'm going to ask my crush to go to the dance right after she broke up with her boyfriend
Is that the right moment to ask her out? Do you know for sure she is single now?
I heard she was looking sad. I asked her about it and later she told me that she broke up with her boyfriend.
Your crush just suffered from a breakup so it is highly inappropriate to the dance in this situation. You should leave her alone for a while so she can heal from her breakup.
I'm concerned about her.
Your concern for her is admirable but give her some space first so she can recover her emotions. In time, she'll see what a great person you are and then would be the appropriate time to ask her out.