If Carlos Danger turned it over and gave them consent no warrant is needed. I think that is what everyone is missing here.
I assume the FBI was just being diligent. If the computer was Huma's, they may need her permission, something they probably didn't have. Can you imagine if they accessed the computer based on Weiner's approval and later it came before a judge (appointed by Clinton) arguing it was Huma's computer and poisonous fruit?