My best friend is not treating her boyfriend, who is my friend as well, right. What should I do?

The best thing you can do is just try and stay out of it. I know you hate seeing your friend being treated badly but it may end up you being hurt from it. Eventually, Austin will figure out what she has been up to. At the end of the day, it is between the two of them and you have to let them sort things out for themselves.