I am going to try and convince my friends that if they cut their hair shorter they will look much better.
Why do you say that?
I hate my friends hair style
Are you sure they are unhappy with their looks currently? It is kind to help them feel better about their looks if that is actually how they feel.
The main idea is that she had the guts to get this haircut
Well maybe you should talk to them about it before saying they should do it. They may really like the way their hair is now, and them getting their haircut and looking better is simply your own opinion anyway.