Well, your best friends right? Then you two should be able to talk about anything and everything. Real friends tell each other things, whether they like it or not especially if its in their best intererest. Explain to your best friend that you don't like the way they are acting and something should be done about it. As best friends, you will discover that it will be ups and downs and even arguments you may get into which may be a little crazy and when you'll both look back on what the argument was about, you realize that it was just a waste of time. Life is too short to be worried over pety things. So when you talk to your best friend, be straight forward and hopefully you'll will come to a compromise.
Tell them and explain...if your best friend is a guy, tell them you hate them... They will get the picture... And ignore them...it takes that with guys
If your best friend is being a brat over something stupid then talk to him or her and tell them that they are acting spoilt over something really stupid! It takes that sometimes to get them back to their senses. It works for me!
You should talk about how you feel nicely don't insult them at ALL! It'll just make it worse,so talk it through and tell them if theyre really your friend theyll understand,if they understand give them space to think about their behavior