From a person perspective, my boytwin almost died as a newborn (in part because I didn't notice how ill he was) and was very ill on and off for ages afterwards. I became very protective of him until one day someone sat me down and was excruciatingly blunt with me about the effect this favoritism was having on my other kids. I'm glad she told me and perhaps your friend will be glad to but be prepared for her to be very offended, at least initially, because she almost certainly hasn't noticed her own behaviour.
This is why you should. Because I would want someone to tell me. And I'm grateful to my friends who do. People don't do things like this knowingly. And you could be saving a family years of rebuilding when this stuff comes up as adults.

And trav you're awesome.