She's not trying out bisexual, she is bisexual.
Let me give you some perspective from another point of view:
I'm bi-sexual. I was a normal little girl (well, as normal as a kid is, anyhow. I was hyperactive and silly too.) I loved the outdoors and playing outside. I wasn't in to fashion, or any of that, but not every girl is. I had friends and was just a kid. When I was in Jr.High, I had a crush on a couple of boys, but found myself wanting to be closer to girls. I thought nothing of it. By high school, I started finding girls attractive. I developed a crush on a female teacher, and a friend of mine. I was mortified. I knew my parents would never accept such a thing, as they thought of homosexuality as an abomination. This is when I realized that I could never be accepted nor loved by them for who I really was. So, I had to hide it and keep it a secret. Everybody I knew made fun of homosexuals, and nobody would ever accept it. I had NOBODY to turn to for true acceptance and understanding. I was alone in the world, as many teens find themselves. I developed a severe hate for who I was, but I could not stop myself from wanting to be in a girl's arms. I could not stop myself from having a crush on a few girls. Honestly, I couldn't understand how other girls couldn't feel the same way. At some point, because I felt so alone and unaccepted, I started attempting suicide.
At some point, I dated guys and girls, and eventually dated and fell in love with a girl. The relationship was broken off at some point, when she did a few very mean things.
When I was 22, I met a guy who I fell in love with. 2 years later, I married him. We've been married for 14 years now and have two kids.
I still find girls attractive, just like a heterosexual woman still finds guys attractive. I'm not gay, I'm bisexual. This means that I had more to choose from.
The reason I am sharing this story is because of the confusion that teens face when they are not heterosexual. They are not accepted and feel like outcasts. If they are not accepted by their parents, they feel completely alone and unloved. This leads to depression, which is already a huge problem among teens. It leads to self hate. It hurts more than anything. If your own parents can't accept who you are, there is nothing to live for.
It's not the end of the world. If her parents had all kinds of dreams of her growing up and marrying a man, they might have to re-think their dreams. She could end up marrying a woman - or a man. Let her find her happiness, and if you do nothing else, give her love and acceptance. She might open up eventually and have some great stories to tell. She might grow up to have a strong relationship with her parents and family. You cannot control a person's sexual preferences. You cannot change them.