I don't really feel like me applying my experience (and an experience shared among many young women) to this very similar scenario is projecting. This aspect is an entirely valid thing to think about while trying to gauge whether or not his friend is creepy. I literally said he might be an exception, and I am simply suggesting that he observe the relationship to see if he thinks its healthy.

Also: "I want to be there for my bro and he hasn't dated someone in a long time so he is pretty deep already."

He appears to be having difficulty finding a woman his own age so he is getting "pretty deep" with a teenager. Red flag.

Also it's great that you know girls in their early 20s who have their shit together, but 19 is not early 20s. Those few years were like an decade of growth for me.