I had a friend who swore up and down her kid was fully potty trained at 9 months.

Bullshit.

Mom was trained. Mom had figured out when the kid went and made sure to put her over a potty at the right time. This was to avoid the kid screaming bloody murder because they had chronic, really bad diaper rash.

Until a child can recognize on their own without being asked that they have to go to the bathroom, walk to the potty unassisted, remove their clothes unassisted, go to the bathroom without missing the target by themselves, clean themselves off as needed unassisted, and put their clothes back on appropriately they're not potty trained.

Potty trained means you can do all the things to go to the bathroom by yourself consistently and correctly.