You'd be surprised. Girls definitely notice the muscles. :)
You know the joke where people teach you how to be attractive and the first thing you need is "be handsome"? It's similar to lifting weights. You only get the confidence after people find you attractive.
My only point really is that they/whoever needs to find something that boosts THEIR confidence and not blindly follow things that worked for someone else in their specific situation.
Plus there is an obvious issue of people putting their self esteem focus on something else they have little actual control about. That's not a sustainable solution on its own.