I an empathise with wanting to be straight. I don't think that sexuality is fixed, especially if you are bisexual. You might find men more attractive, interesting, ect at one time and women at another. I do think that when you are in love it is with the person and not their gender. People realise that they have same/opposite sex attractions at different times so I do think that they can become bi. Some people define their sexuality by who they are with (so if with a guy they are straight), your friend might be doing this. Whether it is an authentic reflection of her true self is another matter but bisexual is just a label.

As for what you should do, I would suggest that you think about your current relationship, whether your guy is ok with you also having a GF. Whether if he isn't you don't want what you have with him. Whether you are just feeling like you want to be with a woman because its what you don't have now and the grass is always greener. I think you look at your relationship and decide whether you want to be with a girl more than you want what you now have. And even if your guy is OK with it, whether you can have a non primary relationship with a woman, because lets face it women tend to relate to eachother on a very emotional level and for me it would be hard to have a non primary relationship with a woman.

one does not wake up all of a sudden and decide you be bi or lesbian... it is always in you, it just make take a while for you to either accept it or for you to even that there is a possibility that you are... i always had the thought in the back of my mind that i was but up until recently (4 months ago)i did not really presue anything into it... needless to say i have been with my girlfriend for 5 monthe on christmas eve... what im trying to say is you do not all of a sudden decide you are this or that... you are what you are and if someone you think is a friend cant accept you for who you are then they are not a very good friend... when you love someone you do not look at them as a male a female a black a white or a mexican... you look at them as a person, someone that you love... not someone who has no feelings or cares...

I do not think it is a matter of no longer being bi or straight.If you are truly bisexual,I do not think you will just become straight.If your friend meets someone she is attracted too,can she tell herself she is straight and ignore her feelings?