The most important thing (in my very humble opinion) is for the child to think they are beautiful themselves, in every way, and to control who gets to evaluate them so that if there ever is a day someone tells them they are not beautiful they know how to evaluate themselves properly (not just on physical beauty).
BUT I do think think that some of how we learn to evaluate ourselves comes from other people at an early age giving us positive messages and if we do not get that we would not learn how to do it.
If that makes sense.
So the 'danger' is that physical beauty is praised more highly than anything else, that someone else gets to evaluate that 'quality' and that the individual hands out that most precious commodity, our love, to people based on what they say about their physical appearance.
But I do not think the friend who posted that comment meant that. I think she meant her child's father thinks their child is beautiful (as does she) and their child loves her father. She may not see anything deeper than that.
I tell my son he is beautiful, and brave and a lot of things, and my daughter too, but I am increasing trying to ask them and encourage them to have a good view of themselves e.g. 'You must feel very proud of YOURSELF for being brave today' etc. "You must know how beautiful you are."
I think the love we feel for parents etc is much stronger than a phrase or word so I am guessing that post is a rather 'cutesy' comment which could be accompanied by a kitten picture and be hanging in someone's loo or the hospital waiting room.
If you want to retaliate I would engage in conversations with the mum around building self esteem for kids etc and not tying love etc into how people look. I think it is a 'parents exploring together' kind of thing rather than a 'there is a phrase I can use to combat this on facebook' kind of thing. If that makes sense.
I do cringe when I see what I consider to be in appropriate stuff on facebook and I agree with you this comment is negative and unimpressive (to me) but I bet others on facebook will like it and think it is lovely and cute!