how about reframing your concern so you don't seem like you are criticizing her?
perhaps telling her you think it's wonderful that she recognizes her own need for sleep and that it is important to take care of herself as well as the baby. sympathize with how hard it is in the early weeks with the leaking and the sweating and the mushy belly and the hair falling out and just plain learning to be a mom, and how you barely remember those first 2 months, etc.
then mention (and have print outs) that babies need to nurse very frequently during the first few months, and that stopping the night nursings this early will very likely lead to a total loss of her milk production. milk production is at it's highest overnight, and the baby needs a substantial portion of it's calories at night.
get her a copy of the "no cry sleep solution" and tell her it is much friendlier to nursing mothers and babies. and let her know that babywise is not breast-feeding friendly. you can follow babywise, or you can nurse, but you ain't gonna be able to do both.
you may even suggest that she pump so her husband can feed the baby once or twice a night, or if she refuses to pump, jesus, let the baby have formula overnight, but it HAS to eat at night. not even ferber thinks night feedings are pointless until 4 months.