I've refrained from commenting here and on the posts on Facebook for a reason: The boy's side is only one side of the story.
Obviously, you should take care when merging or changing lanes, but unless someone cuts into your lane very quickly, it is ridiculously easy to avoid an accident.
Just last month, a truck tried to run us off the road on the NLEX. Twice. Buti nahuli siya sa tollgate, and he was fined thanks to CCTV footage. But that's beside the point. The point is, it was ridiculously easy to stop and avoid a collision because the other vehicle was ahead of me. As seen in the pictures, the boy struck the rear fender of the woman's car.

If their front fenders collided, he could claim that she suddenly cut into him. As it was, she was almost completely in front of him before she merged or she was already far in front of him and he was simply going to fast and failed to slow down in time.
I hear he says he was sitting still? In that case... how do you hit a stationary car with your rear fender? Due to the way steering works on a car, a misjudged swerve will see you hitting an object inside the arc of your turn
with your doors, not your rear fender, as it moves away from the arc of the turn.
Doesn't matter whose attitude you hate more, a case is not decided by emotional appeal, it's decided by facts. In the absence of evidence to the contrary, the girl actually has the stronger case. The angle of her vehicle as it merged is not sharp, and does not suggest she was swerving violently, and she was almost an entire car length ahead of the boy during the collision.