I told my friend that her attire is inappropriate
That could cause a rift. What was inappropriate about her attire?
Her dress was not decent and she wasn't properly dressed
You have a right to voice your opinion to a friend. Just be sure you're constructive and not just insulting them. If not, they may feel upset and may disregard your opinion.
I'm not being kinda rude, I'm just telling her like it is. She's free to wear whatever she wants to.
That's good that you acknowledge that she has the right to wear what she wants. Sometimes it's better to tell someone kindly what you think instead of being blunt. People take kindness more into consideration.