I'll take their word over uneducated ethical argument. I apologize for being presumptuous about your occupation and field of research.
The article stressed that green lighting the research was a hugely controversial decision. I'm also pretty certain there are a huge number of scientists who do research on animals are wouldn't be comfortable with this research.
Also, I think the parent was rejecting the deontological framework in which the decision was made, rather than calling into question the resulting calculation by the ethics board. So an appeal to the expertise of the panel doesn't make much sense.
Therefore, the parent's appeal to the IRB process as the harbinger of ethics is misguided.
If any IRB would deem a study ethical, then there's probably consensus among the scientific community on the cost-benefit tradeoff of the study. If one IRB deemed the study ethical, and if there're probably a lot of other potential IRBs that would not, then appealing to some sort of scientific consensus on the matter is a misnomer.
And besides, animal suffering matters to many people, often more than the answer itself matters.
Of course, it is more hairy than that. There is an estimate of the number of people on the original track as well as a probability that the reroute does not save them.
Even if a new treatment for depression were developed due to this research (which it probably won't), and you were clinically depressed, would you want to receive it knowing it was came about after the torture and eventual deaths of 20 baby monkeys? I wouldn't feel comfortable with that.
Personally, I wouldn't have a problem with taking said treatment, but I know that others may have different feelings about that. The ends don't justify the means is a very valid ethical opinion, how relatively extreme it is in this case depends on the size of the possible award vs. the trade-off, which takes professional knowledge to properly estimate.