One more thing to think about (your friends baby in the onesie) I think of how I get on a hot day, if you have your skin one skin, it gets sweaty. Then if you add the blanket it will make anyone sweat. If the baby is in a full cotton footed pj (you can also get pjs that have the feet open) the baby doesn't have the skin in skin contact which helps eliminate sweat. But as always share your concerns with your babies pediatrician, the nurses at the hospital can be helpful too.
Yes babies run colder than older children/adults. They say a helpful rule of thumb is the baby should be in one more layer than you (mom). You can find light weight footed pjs-cotton is always best. Stay away from synthetic materials (polyester etc.) I also love the thin muslin blankets (my favorite are by Aden & Anais). My 2nd baby was born in a hot Summer, I still kept her in a hat and footed pjs. You don't want the baby to get cold. But if the baby is sweaty just take off one thing at a time, if your baby has a hat just taking that one thing off will drastically change the babies body temp. Newborns are used to being inside the womb were it is VERY warm because of the moms added body temp and just being inside your body.... They sleep better too.
Well I know someone whos baby was sweaty and he had a blanket, onsie, socks
Just keep in mind newborns are a lot cooler than us at first. My daughter had a hard time keeping her temp up. Mid July and she had to wear a hat and be wrapped to go out for about a month. But yeah after that onsies and diapers. Lol