Most Helpful Girl
Her relationship with this new person isn't a rebound since you didn't dump her. I think maybe you said or did something to offend her. Otherwise she wouldn't have dumped you out of no where.
It could be healthy for them because they're together and can physically keep each other company. Not to rain on your parade but the grass might be greener after all.
Most Helpful Guy
Maybe, in the long run, when she reaches the same point with him that she did with you (excitement gone, mystery gone = spark gone) - but more than likely she'll move on to somebody new who isn't you.
Best to just try getting over her, she's one of the ones who thinks that "chemistry" = "love" and will be forever seeking it, not understanding why she can't ever seem to find someone that it sticks with.