I want to date a girl after a mutual acquaintance just went out with her
Oh that is not good. Are you sure this is something you want to do?
Yes, it is my own decision.
You should not date people who are dating others. You can ask them if they were only casual or not interested in the other person.
It's fine, she is just someone who I have a little interest in. If she avoids me that's okay.
If you are friends with this person you shouldn't date someone that they are dating or have dated. I would talk to them about it first to make sure there are no conflicting feelings.