Completely agree with ehBeth.
She certainly has a right to grieve, but not to inflict pain on you through what she is doing or to invalidate your feelings this way. She probably shouldn't have entered a relationship so soon after his death.
EhBeth has some great points about seeing someone together so she actually hears you.
It's sad for both of you and I'm glad you're so understanding. I think I would feel the same way. And it might bother me, too. Not that he would still love her, but that the dead are more important than the living.