I hope Im not going out on a limb her but what culture does your friend come from? My family is Latino and I live in a predominantly Asian and Indian populated area (and have my entire life by virtue of the diversity here in the Bay Area) and these three cultures tend to layer and blanket their infants and children to extremes. As a teacher, I have experienced this also: regardless of what the season or weather is, Mexican, Asian, and Indian parents will often layer their infants, toddlers, and small children in anywhere from 3 to 5 layers of clothing, blankets. I have had many a client and student who, upon arrival at school, has to be stripped of jackets, long sleeved shirts, short sleeved shirts, pants, shorts - almost as if their parents have put every piece of clothing in their closet at once! More than a few times in my professional career I have had to have meetings with parents and families that continue to dress their children in clothing inappropriate for school and play, much less life! If she is from one of the above mentioned cultures, she was probably taught this over layering habit by her own mother and family members, and doesn't even realize it is this what is causing her newborn to sweat. This BY NO MEANS has anything to do with race, they are just cultural norms!
As a rule of thumb, tell her to dress her child as she would dress and blanket herself in accordance with the weather of the day and the season. If he sweats in his carseat, suggest she not put a blanket over him when riding in it, to remove any jackets that he is wearing, and just leave him in his typical little daily outfit - if he seems cold to the touch, cover him. If not, then he is probably fine. Infants DO regulate their own body temperature, unless they have some preexisting condition or brain damage that effects that particular physiological function.
Many parents also use the infant carrier as a sort of "perma bed": if their infants fall asleep in it, they leave them strapped in it to sleep, to eat, to go out and about. Its not really meant for the infant to be in it as often as many parents leave their infants in them, so suggest that she only use the infant seat for car riding, and have her take him out of it and put him in the stroller, sling, bjorn, or whatever she likes to use for transport of the baby when not in the car.
Hope this helps!