I'm going to mess around with a girl my friend also likes.
Why would you do that?
It is easy for me to do and I saw her first.
It's understandable to be attracted to someone, but its rude to take interest in a girl your friend also likes. You should try talking to your friend and seeing if you're okay with pursuing her so that you don't ruin your friendship.
I think I'm still going to mess around with her anyway.
This is a bad thing to have come between a friendship. It is socially expected to not date someone a friend is interested in. You don't want to end friendships this way.