What diet should a mother rabbit be on to gain some weight and support lactation?

I recently began caring my friend's rabbit because my friend was hospitalized. To my surprise the rabbit was pregnant and had 5 kits 2 days ago. I don't know much about rabbits (with the exception of what I have read online) and it seems like the mother is on the skinny side. Is this normal for mothers that have recently delivered and what type of diet should I put her on to get her weight up?? Right now I am feeding her pellets, orchard grass and the occasional store bought treat and sometimes some carrot pealings.