I am ging to sound a different note here - I think there may well be a problem here, and just carrying on doing nothing different is not an option.
This is a baby who appears to be actively losing weight. That is a sign the baby is not being well-nourished. This has nothing to do with the quality of her breastmilk (which will be fine) and nothing to do with how much she can express....and everything to do with the amount of milk getting into the baby.
Firstly, the weights need to be confirmed as accurate. If they are and this baby is still way off birth weight and losing more weight on top of this, then he needs to be seen by someone who knows what they are looking for. This means a bf friendly paediatrician or GP, or a very experienced and knowledgeable bf support person who can recognise when the mother and baby need more help than she can offer.
If it was me, I would watch a feed, take a careful history or what has happened so far, and I would probably be looking for ineffective feeding....I would ask if he is pooing several times a day and more than just a skid mark? If I could spot ineffective feeding, I'd try to help her adjust positioning and attachment, and encourage her to feed more often....but I would still get him checked out by a doc, because this is the sort of profile that fits babies who are actually either ill or who have some undiagnosed congenital problem (eg a heart defect) where all the calories are being used up in keeping the baby well, with none left over the grow.
I don't think this baby should be left for even one more day without a professional's view.
I don't want to panic anyone, but on the facts you have given here, Lisa, this is not something that should be left alone.