What do I do about my best friend?

For the last.. 4 months. My best friend has become a whole new person. She seems awfully pessimistic and depressed all the time. She's always full of negative comments and never looks at the bright side anymore. She use to dress up all cute and do her hair and make-up, but now she just puts it up in a clip, and just wears jeans and a t-shirt. Not that I care about what she wears, but it's so different. I'm afraid it's because of her boyfriend. They argue every time they come over ,and everywhere they go. They argue about stupid things, like water bottles and his car keys, and even just touching eachother. They have been bickering more and more since they moved in together. And she's only 17! Her parents let him move and things have been different. We don't have just me and her time anymore, and she's always so irritable. They pick at eachother, and might I add that he's not the best boyfriend in the first place. Immateur, un-intelligent, and anger problem. Help me please!