I myself believe, 16 could be too young to be 'in love'. Most 16 year olds aren't mature enough to really understand and grasp what being in love is. You can like somebody, you can like them more, you can love them as a person, but more than likely you're not 'in love'.
Be happy for her though! If they seem to love each other so much, and if you loved her as a person, you should be happy seeing her be happy, even if it's with someone else.