It sounds like your friend and this boy like each other. You can't control his or her actions, but you can control your response to what they do. We know it hurts when the guy you like decides to start dating somebody else; especially when she is one of your friends. Instead of becoming jealous, angry, or bitter towards them, you really can choose to take the high road.
If he wants to be with her then that is his loss. It might feel like he is the only boy you want to be with, but that corny phrase of, "there are plenty of fish in the sea" really is true. Plenty of quality guys are out there who can appreciate you for yourself and make you happy.
The way you chose to approach this situation will say a lot about how everybody else handles it. If you choose to get jealous and bitter, there is a good chance that your friend and this boy won't be very happy with you. They may also very well say mean things about you to other teens your age as well.
If you take the high road, you really can avoid these types of things. If these two want to be together then that is their decision. By choosing to take a healthy, drama free path away from this situation you will put yourself in a much better position to feel better much sooner.
It can be tough to cope when you feel let down, so please remember how important you are. Hang out with some friends, listen to music, do art, write in a diary, or do anything else you do which help you cope. You and your feelings are important, so please don't let the stress and discomfort you feel get in the way of your happiness.