I agree, Pruners, that it really is not worth quibbling over the supposed 'best' nutrition when we are talking about 2 weeks...
The 'perfectly healthy' but 'thin' breastfed baby of 5.5 mths probably would benefit from more calories (babies probably shouldn't be thin at 5.5 mths). Giving solids is probably the least efficient way of getting more calories into him, though - solids (as I thought I had explained in my previous post) don't make much difference in terms of calorie intake for a while, unless you deliberately make them very dense, such as giving a lot of baby rice (approximate calorie value = 7 kCal per teaspoon in powdered form) which most mothers (rightly) don't do straight off.
The mum in this siutuation doesn't need to be tearful if she decides to give solids - of course she doesn't. But she does need to know that the easiest, and most nuritionally appropriate way to get a bit of chub onto her baby is to breastfeed more often. She may choose not to do that, and to give solids instead. No big deal, really. OTOH, what would be wrong with explaining to her that getting an extra breastfeed per day down her baby is likely to save her the faff of preparing food for her baby, and give her baby more calories...which is what you are sure this baby needed?
A very small breastfeed of say, 50 mls, will give the baby 35 kCal. A suggested first meal for a baby of this age (I have just checked on the Heinz 'tiny tums' website) is made up of water, 1 courgette, 2 carrots, 2 teaspoons of baby rice. I actually don't think a baby would eat all of this (a whole courgette and 2 carrots??) but even if he did....you do the maths, allowing for 14 KCal for the rice, and probably the same in total for the veg.
Give an extra two small breastfeeds, and that's an extra 70 kCal