I'm upset about my best friend's new relationship
What is upsetting you the most about this relationship?
I found out the guy is a scoundrel and he's not good enough for her
If you feel that way then you should discuss it with your friend. Be careful however to not have the accusation come across in the wrong way. Just do your best to be honest and unbias in the situation.
What should I tell her? Tell her that he's a scoundrel?
I'm glad you realize that you should speak up to your friend. I would try and say things nicely because if you put him down, it would only make her want to see him more.