Checking bags at IAD

Not sure if this should go here or on the Hawaiian board, but here goes...

My daughter will be flying IAD-SFO (on United) then SFO-HNL-AKL (on a separate Hawaiian ticket). Does anybody know whether she will be able to check a bag all the way through to AKL, or will she have to uplift the bag at SFO and re-check it.

Any advice gratefully received.