All ridiculously overpowered.
I disagree completely, and if you actually believe that, then you need to point out why. What you say doesn't help him make them better if there is an issue
I like the concept of trying to revive these cards and print the whole set, first off. I also like a lot of the abilities. Here is my analysis:
Plague of Blood: I like it, I think. Punishes splashiness, and not overly powerful. Don't see any real problem with it, looks good
Plague of Frogs: I'm not sure I'm a fan of this card (it may be the only one I'd consider OP, but it'd be slight if it were), as it is a much more powerful and useful Set Fire. The TC addition makes it a little too powerful for me, and there is no restriction to Exodus, so it is very splashable. Side note: It doesn't stop with one player's cards. It eats ALL sites and evil fortresses, eventually.
Plague of Lice: Interesting card, and there isn't really anything that does it in good, unlike evil. I'm not sure about the protection, perhaps I would have it protected from evil cards instead? That'd also fit with the story.
Plague of Flies: Nice, very nice. Unique ability that is also useful, I like this one.
Plague of Pestilence: Also a unique ability that I like, and the restrictions for Exodus make it less splashable. I would maybe work on the wording of the last sentence, because it may lead to confusion about whether
one EC is reduced 2/2 times the number of artifacts, or X EC are reduced 2/2, where X is the number of artifacts.
Plague of Boils: I like it, more or less, at a first glance.
Plague of Hail: I actually don't like this one very much, mostly because negate does not actually target, by definition, which makes the SA a little awkward to implement.
Plague of Locust: I like it, a fun card to put into a deck like this.
Plague of Darkness: Really like this card and would use it. Very good special ability that is both useful and balanced.
Death of the Firstborn: Decent card with a good ability.
Let My People Go: You could forgo the limit by just changing the wording to "On activation, if you control..." That would automatically limit the use to when it is activated, like most other artifacts with abilities like this one. Otherwise, I like it a lot, as it only searches deck, requires the use of Aaron/Moses, and is limited to a certain type of card. Not sure why you would really need the CBN, unless you were intending it to be able to be used in battle. In which case, I'm not actually sure the current wording allows that. You would want something like "Once per turn on your turn, if you control..."
The reprints on the artifacts are good, and I like the new version of Aaron's Rod.