Hello!
I can't tell you how many times I've had this same conversation with my mother and I have a DEFINITE opinion for you! :-)
The short version - Please allow your friend to make her own decisions regarding her son. Mothering is hard, as I'm sure you know, and she doesn't need you second-guessing her. You do NOT get a cold from BEING cold, you get a cold from being exposed to a virus, which can happen anywhere regardless of whether or not you're wearing a snowsuit. That would be why people get "colds" in the summer!
Here is why I feel this way:
My daughter is 15-months-old, but tiny so she is still in an infant carrier. The first really cold day, I happily got out her pretty, puffy pink winter coat, bundled her up, and put her in her car seat. The problem? I couldn't tighten the straps. At all. I was torn between the pretty winter coat and my daughter's safety for about 2 seconds, then took the coat off, put her light-weight sweatshirt on, fastened her securely into her seat and put a comfy blanket over her! I pre-heated my car and she was FINE. When we got to the store, I kept her in her carrier under her pre-heated toasty blanket for the 15 seconds it took me to walk her from the car to the store. She was FINE.
This entire time, however, every time my mom saw her she made "helpful" comments and remarks such as, "Oh, poor girl! Grammy will buy you a winter coat! Mommy just doesn't realize how COLD it is out there for a little one..." These offended and upset me to no end. It eroded my confidence in my decision and made me feel like I had to "hide" my daughter from my mother. Finally, a few weeks later, we were going to my parents' church and I gave in and put her in the stupid coat so I wouldn't have to hear the comments from my mom. As we were driving home, we were rear-ended in a car accident. I cannot possibly tell you how guilty I felt that I had selfishly given in to avoid my mom's pointed comments and THAT was the day that my little girl was in an accident in a coat that offered almost no protection from her (and her brain, internal organs, etc.) jiggling around in the event of an accident.
Also, because of the accident she was in the coat in the seat for quite some time while we exchanged insurance/contact info, etc., with the other driver. When we got home and took her out of the seat and coat, she was SOAKED with sweat. I took her temperature and it was 102.4 and she had a glassy-eyed look. Once she was out of the coat the temp went right back down to normal and she just couldn't drink enough water. My poor baby was overheated, dehydrated, and had just been less-than-protected from a collision because I couldn't stand up to my mom.
I hope you're able to put your worries to rest and trust that your friend is doing what she feels is best for her child. It is not harming him, so kindly allow your trust in her to override your need to comment!