Is she a first-time mom? If so, I think it would definitely be helpful to have at least a couple nursing tops & nursing gowns in her wardrobe. I didn't particularly care for any of the nursing shirts or pajamas that I had, but I did feel more comfortable nursing in them initially.
All of the ones I got were from Motherhood Maternity. I know Old Navy has some so they're probably more reasonably priced, but I don't know about quality.
After getting comfortable with nursing, I'd just wear a comfy sleep nursing bra to bed at night underneath my normal pajama shirts. During the days, I'd wear a nursing tank under my normal shirts so I'd have full coverage over my stomach & everything else when I lifted my shirt.