וְלֹֽא־עָצַ֧ר כֹּֽחַ־יָרָבְעָ֛ם ע֖וֹד בִּימֵ֣י אֲבִיָּ֑הוּ וַיִּגְּפֵ֥הוּ יְהוָ֖ה וַיָּמֹֽת׃ (פ)
Neither did Jeroboam recover strength again in the days of Abijah; and the LORD smote him, and he died.
Rashi on II Chronicles
And Jeroboam could muster no more strength, etc., and the Lord smote him i.e., Jeroboam.
Rashi on II Chronicles
and he died He did not die immediately, but continuously deteriorated because of the wars, and in Genesis Rabbah (65:20), it is explained that the Lord smote Abijah.