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Yes, you can argue that. The rules are pretty clear that all defendants must join removal, and that they have a specific time frame to do so. 28 U.S.C., Section 1446 really doesn't leave a lot of room for interpretation:
(1) The notice of removal of a civil action or proceeding shall be filed within 30 days after the receipt by the defendant, through service or otherwise, of a copy of the initial pleading setting forth the claim for relief upon which such action or proceeding is based, or within 30 days after the service of summons upon the defendant if such initial pleading has then been filed in court and is not required to be served on the defendant, whichever period is shorter. The statute also says "all defendants who have been properly joined and served must join in or consent to the removal of the action."

"Shall" is mandatory language. Unless that last defendant can show that he didn't have any notice of the suit, and no one told him, you have a good basis for disputing the removal. If that defendant wasn't properly served, that would hurt your argument, and they might be able to get the case moved. That's the only argument that should let them get back to federal court, if it's based solely on diversity of citizenship.