Other mistake: Marta sinks her white shoes in the mud to cover her tracks. We see that the shoes are, as they should be, all dirty to the sole and welt. They are clean again in the following scene though (on a sidenote, the shoes have been cleaned after the murder then, and she or her mom missed the bloodstain - it's very much possible, it just adds a little implausibility). (00:57:20)
Other mistake: During the revelation of the second death, when the killer opens the toxicology report they just retrieved, the date is revealed; it says it was issued on 09/27/2018. But the events of the movie happen in November (it's Sunday 18th Nov when Marta wakes up with all the reporters at her door). It should also be noted that when a close-up of the report was shown earlier (when Ransom was looking at the anonymous letter brought by Marta) that copy had a big stamp in a corner saying 11-17-18, that in this copy is totally absent. (01:56:55)
Answer: She didn't drain either of the vials so unless he actually measured how much of each liquid there was to start with, he was unlikely to notice that the amounts weren't quite what would have expected. He knew enough to know that the switch could lead to a death, but he's not anal enough to note every tiny aspect of the situation to verify that nothing's gone wrong.
Garlonuss ★