Nipple sheild could work, i also found that after i stopped bf and was expressing they hardened up loads.
I know you say she doesn't want to give formula, and i'm sorry if this is contreversial, but if baby isn't happy and isn't puuting on weight then formula feeding is best for the baby.
My DD lost a lot of weight because i was determined to bf, but this was me trying to satistfy my wishes and feeling pressured by society and the breast is best issue. it wasn't best for my baby cos she wasn't gtting any milk. I firmly beleive mum should do what is individually best for her baby, and prime concern should be them gaining weight.
She could offer some formula to get the weight back up, keep pupming to sort her nipples out and then go back to bf when the situation has improved.