I would be inclined to agree with you and your friend... Your puppy does not look like a boxer. Pits and boxers have very different head structure, and your little girl appears very much to have the scissor bite and elongated muzzle of that breed - rather than the square head of a boxer, with undershot jaw. She is also missing the essential black mask.
It is
possible that she's part boxer. But I would find it very difficult to conceive that both parents are full-bred boxers.
I understand that she is registered as a boxer with the AKC. But that doesn't actually mean anything more than whomever bred her stated that her parents were dogs also registered as boxers with the AKC. This is an honour system, after all - it is never checked, unless someone files a complaint (in which case, an upheld complaint may end in the breeder being 'struck off' - so they'd use some other registry instead). To put things into perspective, people have successfully registered kittens as pure bred dogs (just for proving the point, I think). The AKC is, after all, nothing more than a registry.
None of that changes that she is beautiful, of course

And I'm sure she is going to be a wonderful pet, whatever her parentage. But you are right to note that she looks nothing like a boxer, and it's unlikely that she is fully of this breed.