Firstly, I would disapprove of the friend, so let's get that out of the way first.
However, the issue with a lot of these studies is that they either are not properly controlled or they are conducted in extreme circumstances- a lot of the evidence comes from the famous Romanian study which cannot be repeated as it would be deemed unethical, or from observation of babies living in extremely emotionally deprived institutional environments (such as Chinese orphanages in the 1990s). Even then, there is also physical deprivation going on (and incidences of blatant cruelty), so it's hard to untangle the causes.
Therefore, it's not possible to correlate the impact of, say, leaving a baby to cry for an hour, vs. being left in a cot all day with zero responsive care, zero stimulation and zero interaction, which is what the study subjects were typically exposed to.
Extrapolating to "normal" circumstances, where a single parenting style is not great, but all the rest is fine, is usually therefore pretty alarmist.
e.g. all this bollocks about needing a parent facing pushchair or your child won't learn to speak. Extrapolation at it's worst.
There is also evidence that other accepted practices, such as nursery care for babies < 1 year, and living in a blended family environment raises cortisol levels in children, but, you know, people pick and choose which parenting practices they disapprove of, subject to their own circumstances.
Also, when people say that cortisol "damages the brain" what they mean is that it can lead to anxiety/depression because it can lead to a situation where the fight or flight response is activated too readily basically and overpowers the rational brain.