Been thinking about this since you posted ... it's one thing to let your friend know that you are there for her but if she is in the dark about the truth there is potential for big miscommunication and hurt feelings down the road. She is going to be hurt no matter what, whether it's by DH or whether it's because she feels a need to shoot the messenger.

Can you work with her cousin to talk with her together, get the facts out in the air? If it's two of you ripping off a band-aid rather than just one, you may be able to present a united front in delivering painful information but still letting her know that you are on her side.