Hi, princessproudmel. Sorry about your friend's difficulties :(
Unless your friend's baby was extremely big, then this situation is concerning - can you work out exactly what percentage of weight he has lost? It's probably easier in metric :)
The midwife may well be right that the baby needs topping up at the moment and your friend really should listen to this - but she is WRONG that eating more high fat/rich foods will make a blind bit of difference :( :(
If babies have lots a lot of weight (and this should have been spotted by the midwives days ago, seriously - a baby should not get to this age before poor feeding is discovered) then they can get to a point where they don't suck effectively, and they conserve energy by staying asleep a lot.
Formula can be necessary, for a short time.
However, at the same time, if she is keen to bf, she needs to preserve her supply by expressing in whatever effective way she can - hand or pump. This should be done many times in 24 hours - if the baby is not bf direct very well, she will need to express at least 8 x in 24 hours, inc at night.
She can have the baby close to her, skin to skin, so she can respond quickly to any slight feeding cue. This is hard, at the same time as expressing frequently, I know :(
She can eat and drink to hunger and thirst - no need for anything special or frequent. This is for her own comfort. Will make no diff. to the bf.
Has your friend contacted any of the bf helplines for real life support and info?