all of his work is peer-reviewed by other top academics

Bart’s academic publications are peer reviewed (not necessarily by “top” academics but by academics judged qualified by the editor who also happen to be willing to undertake thankless unpaid work). I seriously doubt his popular works are peer reviewed.

how can one show an apologist with a mile-high stack of fluffy devotional mindless bull that an Ehrman book is categorically more reliable as a discussion of the Bible?

You almost certainly can’t, without first dismantling his world-view. People are apt to assess authority through a lens of tribalism: Because Bart is an agnositc, a professional academic at a secular school, unless he happens to agree with your friend on controversial points (in which case he will be used as a token–“Look even this faithless scholar admits X!”), he will most likely be dismissed as the enemy.

surely there is some objective standard of reliability that UNC and/or the book publisher would require of “Armageddon” that a publisher of “The Case for Christ” would not use?

Bart is tenured. UNC isn’t deciding what he can and can’t publish. Even if he weren’t tenured, they wouldn’t be directly dictating what he could publish (most colleges and universities respect academic freedom enough not to censor their professors outright), but they would have influence insofar as they could decide to let him go if they didn’t like what he had been publishing, so, pre-tenure, he would probably try to steer clear of anything that might make too many waves.

I can’t say how exactly Simon and Schuster review books, but they are a popular, not an academic, publisher and would presumably not use peer review. In this respect they are just like Random House who run the Image imprint, under which the Case for Christ was published.

The more objective standard is to read the arguments dispassionately, to assess the evidence on the merits without bias.
Of course that is easier said than done, and that is the bitch of the thing.

I mean, you can look at an author’s overall standing in the field–who is tenured where, who publishes where, who is taken seriously by the people who devote their lives to this stuff and who is written off as a quack by the same–that is generally a pretty good standard, but the problem comes when you admit that the profession (or the mainstream within that profession) could itself be biased. But that is always a problem when you have to judge positions you aren’t competent to judge on the merits: what if the fringe minority position is correct?