IRB-questionable research from places that does not require IRB process: How publishable/contributing?
It is (or should be) a principle that any research involving human (and some animal) subjects should be subject to prior independent ethics review, such as is done by IRB structures. If you are reviewing for a reputable journal, I would expect (or at least hope) that the journal has such a requirement on anything they would consider publishing.
Whether they have such a requirement or not, you would be wise to note the lack of a statement of ethical review in anything that you examine. Let the editor know that there is a missing, ethically required, piece.
If you seem sure that no review was ever done, rather than just a missing statement, then you would probably also be advised not to review the paper further. Ethical constraints on such research isn't something that can be patched up after the fact. People have died. People have died horrible deaths after long illnesses. Doing such research without independent prior review is not ok.
Note, of course, that there are some kinds of research that don't require IRB approval. But it is usually an ethical board that makes that determination. See this for example.