They are both college students and she can not support him on her own.
This is the problem with the word "sponsor". Your daughter is the only one who can file for his green card, i.e. sponsor him. But you can be a "co-sponsor" for the affidavit of support since she does not have enough income. I hope that clarifies things. Good luck.
Serve as a joint sponsor for Affidavit of Support purposes? Sure. Petition for him? No. She needs to do that.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
As a wife of the Nigerian citizen, your daughter must be the petitioner and primary sponsor. If your daughter does not have a sufficient income to be the sponsor, you can serve as the joint sponsor for them and supplement the missing income with yours. Please, note that under the recent policy, USCIS, but especially Department of State (DOS) employ a very strict view of individuals who show the risk of becoming a "public charge" even where a joint sponsor is involved. Do further research to be sure that your son-in-law's case does not appear as such, because US Consulates these days do not hesitate to deny cases when they detect that risk.
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