Can I show my co-author the final version of an accepted publication I reviewed?
No, you cannot. Until the paper is publicly available you cannot show it to anyone nor even acknowledge its existence. The only slight exception I would make is that if a colleague was going to pursue similar/identical research, I would tell them they might want to contact the author. I would assume that this would result in my identity as a reviewer being revealed.

Your options are:


Submit your manuscript as is, but add the citation as soon as it become public.
Reveal your identity as a reviewer and ask the author for a preprint.


Option 1 is reasonable since you believe the existence of the paper is not critical to your paper. Option 2 is okay, but remember the reviewers of you manuscript will not have easy access to the cited material.