Hmm
When I gave my two treats, I would make a point of giving Lil Bit hers first and then Beamer is, so Lil Bit would know she was "top" dog and wasn't being replaced by Beamer. Now she doesn't mind who gets the treat first, they have settled down together.
So, I think your friend needs to show Dia that she isn't being replaced in her affections. She needs to make sure that Dia is first in line for attention, then make a fuss over her new puppy. Dia should see that the new one isn't a "threat" to her, and they should get on well together.
As it has only been two weeks I should give it some time.
